GRE gen and inorg

Geometric Identity & Hybridization

#e-groups: 2 geoID: linear hybridization: sp #e-groups: 3 geoID: trigonal planar hybridization: sp2 #e-groups: 4 geoID: tetrahedral hybridization: sp3 #e-groups: 5 geoID: trig. bipyramidal hybridization: sp3d #e-groups: 6 geoID: octahedral hybridization: sp3d2

Which 2 electron conditions result in the greatest chemical stability?

(1) All electrons are in the lowest energy orbitals available; a condition known as the ground state (2) All electrons are associated with a closed shell. These closed shell conditions are referred to as an octet when considering s and p valence electrons

8 Rules for Assigning Oxidation Numbers

(1) Any *free element* or atom in its *elemental state* has an oxidation number of *0* EX: Fe, Na, H2, O2, N2, H, O, N have ox# = 0 (2) The oxidation number of any *single atom ion* is equal to the *charge on the ion* EX: Na+ has ox# = +1. Cl- has ox# = -1. Mg+2 has ox# = +2 (3) *Hydrogen* has an oxidation number of *+1* in all compounds except *metallic hydrides* (a metal + hydrogen compound) in which it has an oxidation number of *-1* (4) *Oxygen* has an oxidation number of *-2* in all compounds except for *peroxides (O2-2)* where its oxidation number is *-1* and *OF2* where it is *+2* (5) In combinations of *nonmetals that don't involve H or O* the *more electronegative atom - that's the one furthest to the right or up* is assigned an oxidation number that has the *same value as the charge of its most commonly encountered anion* EX: In SF6, F ox# = -1; S ox#= +6 BUT in CS2, C ox# = +4 ; S ox# = -2 -CS2 is an example of a "borderline case" because C is above S, but S is to the right of C on the periodic table. S is assigned the common negative ion charge because it is further to the right of C (2) than C is above it (1). (6) The algebraic *sum* of the oxidation numbers of all atoms in the formula of a *neutral compound must be 0* EX: In PCl5, Cl ox# = -1, so P ox# = +5 (7) The algebraic *sum* of the oxidation numbers in the formula of an *ion* must be *equal to the charge of the ion* EX: In SO4 -2 ; O ox# = -2 (since O is not in a peroxide or OF2) so S ox# = +6 to make ox#s sum to charge -2 (8) In chemical reactions the *total oxidation number is conserved*

2 Steps to Determine Molecular Shapes

(1) Count the total number of electron groups, both bonding and nonbonding (remembering that double/triple bonds count only once). Use this number to determine the geometric identity. (2) Determine the overall shape of the molecule based on the number of nonbonding pairs Keep in mind that the geometry =/= shape in most cases. The geometry of a molecule is identical to the shape only if the central atom has 0 nonbonding electrons Nonbonding electrons, like any others, want to be as far away from others and in their own space so they push away from other electron groups, creating these different shapes

The 4 Types of Crystalline Solids

(1) Ionic Solids Ionic solids are made up of positive and negative ions arranged into regular arrays where each ion is surrounded by ions of the opposite charge (NaCl, CaF2). Unit cells are held together by Coulombic forces that act between cations and anions. Ionic solids are hard and brittle, and have high melting and boiling points. They are also poor conductors of electricity/heat. (2) Covalent Solids Covalent solids are made of atoms that are held together by very strong covalent bonds (quartz, SiO2). Covalent solids are very hard and have high melting points. They are usually poor conductors of heat and electricity. In some cases, different forms of covalent crystals can occur that are made up of the same element (graphite and diamond are both carbon based covalent solids). (3)Molecular Solids Molecular solids are made up of neutral molecules (water, sucrose, I2, P4) and they are held together by dipole forces or london dispersion forces. Molecular solids are soft and are poor conductors of electricity and heat. (4) Metallic Solids Metallic solids are made up of one closely packed metal element. Each lattice point of the unit cell in metallic solids is occupied by a metal cation (that's the metal atom without its outer shell electrons). The free valence electrons circulate around the metallic cations, and this makes metallic solids very good conductors of electricity and heat. They may be hard or soft depending on the element, and have variable melting points The structures of many solids can be described in terms of the regular stacking of spheres that represent the ions or atoms.

Three Steps to Devise Equilibrium Expressions

(1) Write out the equation: K = [Product C] [Product D]/ [Reactant A] [Reactant B] (2) Substitute the number 1 for any molecule that is a solid (s), a liquid (l) or the solvent (3) Use the coefficients from the balanced chemical equation as exponents for their respective species in the equilibrium expression EX: PbI2 (s) <--> Pb+2 (aq) + 2 I- (aq) K = [Pb+2][I-]^2 HCOOH (tol) + CH3OH (tol) <-> HCOOCH3 (tol) + H2O (tol) K = [H2O] [HCOOCH3] / [HCOOH] [CH3OH]* *H2O appears in this equilibrium expression because it is not the solvent, toluene is. In this case, H2O is not considered a liquid either since it is dissolved in the toluene, meaning that it is relevant to the equilibrium like any other molecule

Calcium Binding Proteins

8, 7, and 6 coordinate calcium is found in many so called calcium binding proteins, such as parvalbumin, troponin, staphylococcal nuclease, thermolysin, and concavillin. They have various functions, including acting as receptors for intracellular calcium and structural intermediates for enzyme catalysis.

Metals & Band Theory

A metal has an overlapping Valence Band and Conduction band. There is no energy gap to be crossed, meaning that electrons are promoted to the conduction band without any input of energy. Electrons are thus free to move through the Conduction Band of a metal, allowing for the conduction of electricity. Unlike semiconductors, the conductive properties of metals are decreased by increasing temperatures.

p-n Junctions

A p-n junction consists of a p-type semiconductor (which has positive hole charge carriers) that is in close contact with an n-type semiconductor (which has negative electron carriers). At the interface between both types of semiconductors, a few electrons from the n-zone migrate spontaneously to the p-zone. This migration confers a negative charge to the p-zone (due to the gain of electrons), and a positive charge to the n-zone (due to the loss of electrons AND the holes that are created in the low energy MO by the electron loss). This accumulation of charge is known as a contact or junction potential.

p-n Junctions as Rectifiers

A rectifier is an electrical device that converts an alternating current into a direct one by allowing a current to flow through it in one direction only. p-n Junctions make excellent rectifiers because when they are inserted into a circuit in which the electrical potential alternates continually, a p-n junction allows the current to flow only in the direction of direct polarization, thus an alternating current (AC) can be transformed into a direct current (DC). Vacuum tubes were used as rectifiers prior to p-n Junction technology, but they were large, fragile, and unreliable. Today all solid state electronic components in electronics make use of p-n junction technology.

Semiconductors & Band Theory

A semiconductor has a filled Valence Band and and empty Conduction Band just as an insulator does, but the energy gap between them is much less significant. This smaller gap makes semiconductors an intermediate between conductors and insulators and also means that adding energy (such as heat or light) can excite electrons, making them able to jump from the Valence Band to the Conduction Band. As the amount of energy increases, more electrons are excited allowing for better conduction. This allows for better conduction under higher temperatures (the opposite is seen in metals) Thus, due to this smaller energy gap, semiconductors do not readily conduct electricity, but are able to if an appropriate amount of energy is supplied (often by thermal means, or a moderate electrical field).

Dynamic Equilibrium

A system in dynamic equilibrium is one where individual molecules continue to react but there is no net change in amount of either reactants or products. The rates of both the forward and reverse reactions are the same for a system in dynamic equilibrium. Chemical systems in equilibrium will remain in equilibrium indefinitely unless they are acted upon by an outside influence. This happens because many chemical reactions are reversible. 100% conversion to products does not occur for reversible reactions due to the competing reactions that revert products back to reactants.

p-n Junctions as Transistors

A transistor is a semiconductor device used to amplify or switch electronic signals and electrical power. An n-p-n transistor is made by inserting a p-type semiconductor between two n-type semiconductors (creating two interfaces) and attaching the negative and positive poles so that one applied potential increases the supply of carriers for another potential. An n-p-n transistor inserted between two circuits can then take the current in one circuit and produce a proportional current in the other one at a higher power level, thus acting as an amplifier.

Cation/Anion Effects on Atomic Size

Adding or removing an electron from an atom has a dramatic effect on atomic size. The general rule when comparing atomic and ionic radii is: +cation+ < neutral atom < -anion- In general, adding electrons (forming an anion) leads to increased atomic size (may be due to increases in electron-electron repulsion in the valence shell), while removing electrons (forming a cation) leads to decreased atomic size (may be due to emptying of the outermost energy level).

The Octet

An atom has a complete octet when the valence s and p shells have been filled with the required (2+6) 8 electrons, thereby assuming the same configuration as one of the chemically inert (due to octet stability) noble gases. This stability is the reason other elements attempt to emulate the noble gas configuration through disposal or acquisition of electrons. Atoms will always attempt to attain noble gas configuration by the most direct means possible.

Electron Configuration & Stability

An element's chemical properties are exclusively determined by the stability of its electron configuration. Unstable configurations are characteristic of highly reactive atoms while stable configurations lead to chemical inactivity. Since the main goal of any atom is to become stable, an element's chemical behavior is governed by its desire to maximize stability.

Extrinsic Semiconductors

An extrinsic semiconductor is a substance that is normally an insulator, but can become semiconducting if small amounts of other atoms (impurities) are introduced into the lattice rendering it impure. Dominant charge carrier concentrations of either electrons or positive holes in an extrinsic semiconductor are what classify it as either an n-type (electron dominant) or a p-type (positive hole dominant) semiconductor

Insulators & Band Theory

An insulator has a full Valence Band, and a substantial forbidden energy gap between this Valence Band and the next vacant higher energy band, the Conduction Band. This large separation is too much for electrons to jump under normal conditions, and thus the electrons in insulators are usually unable to cross the forbidden energy gap and enter the Conduction Band for conduction of electricity. However, if a sufficiently large amount of energy is provided to promote the Valence Band electrons all the way across the forbidden energy gap to the Conduction Band, an insulator can be made to conduct electricity. For example, the carbon of diamond takes about 120 kcal/mol of energy to overcome its insulator properties and start conducting electricity.

Intrinsic Semiconductors

An intrinsic semiconductor is a solid in which the band gap is so small (and can be made even smaller by adding impurities, known as doping) that some electrons from the Valence Band occupy energy levels in the Conduction Band. This slight electron population in the Conduction Band results in the introduction of negative carriers (electrons) into the upper level, and positive holes in the lower level. Due to this, the solid is conducting. The electrons in the upper level Conduction Band are able to move freely, and the positive holes left in the lower level Valence Band move in one direction as electrons jump to fill them from adjacent bonding pairs in the opposite direction At room temperature, a semiconductor is much less conductive than a metal because a relatively small number of electrons are excited (because higher temps lead to more excited electrons) leading to a smaller number of positive holes and electrons to act as carriers

Electron Affinity Trend

As you move across a row from left to right or up a column in the periodic table, the electron affinity increases, meaning the EA values become more and more negative. The largest -EA values are in the top right and the smallest -EA values are in the bottom left. *Noble gases, Mg, Be are excluded- they have +EA values that do not fit with the overall trend

Atomic Size Trend

Atomic size increases as you move down or to the left in the periodic table. The largest atoms are the ones in the lower left of the periodic table The smallest atoms are the ones in the upper right of the periodic table (the smallest atom of all is simply H)

Atomic Size

Atomic size is a function of electron configuration. It is generally true that atomic size increases as you move down a column (family) because the core electrons come between the nucleus and the valence electrons, shielding them from the strong positive pull of the nucleus protons. It is generally true that atomic size decreases when moving left to right across a periodic row. This is because electrons are added to the same valence shell as you more to the right, but the charge of the nucleus is increasing as more protons are added (seen in atomic number Z). This increased positive charge in the nucleus means that the valence electrons are being pulled in more and more tightly by the nucleus as you move to the right, resulting in a reduction of atomic size.

IMF Strength Domains

Because the range of intermolecular cohesive forces is so wide, the broad IMF category has been divided into three smaller domains according to IMF strength: (1) Strong IMFs: Ionic Forces (2) Intermediate IMFs: Dipole Forces (3) Weak IMFs: Dispersion Forces

Volume of Body Centered Cubic Unit Cells

Body centered cubic cells (bcc) have one sphere in each corner and one in the middle of the cell. The sphere in the middle prevents those in the corners from touching one another. Instead, these spheres touch eachother along the body diagonal. The length of this diagonal is 4x the radius of the sphere. The length (L_bcc) of the edge of the body centered cubic cell and its volume (V_bcc) are related to the length of the body diagonal.

Biological Roles of Mg+ & Ca+2

Calcium is the main component of bone and teeth Critical for blood coagulation and transmission of nerve impulses, Mg+2 is an enzyme activator, and Ca+2 acts as a trigger in nerve signal transduction.

Consequences of Lattice Energy on High Oxidation Number Stabilization by Small Ions

Cations with high oxidation numbers are stabilized by small anions. The smaller the anion (and thus the larger the difference in lattice energies), the better the stability. For example, the only known halides of Ag(II), Co(II), and Mn(II) are fluorides. The heavier halides of metals with high oxidation numbers, for example copper (II) iodide and iron (III) iodide simply decompose at room temperature. EX: A redox reaction where X is a halogen MX + 1/2 X2 --> MX2 -The conversion of 1/2 X2 to X- is more exothermic for F2 than for the heavier halide Cl2, and the lattice energy plays a major role in this. -In the conversion of MX to MX2, the charge of M goes from +1 to +2, meaning that the lattice enthalpy increases, according to the Born-Mayer equation. -As the radius of the anion increases, the difference in the two lattice energies diminishes (decreases in thermodynamic stability), so in order to have an increase in thermodynamic stability, the radius of the anion must decrease.

Cell Membranes

Cells are surrounded by a membrane barrier that separates their interior and exterior. Some substances can be produced inside the cell and need to be exported through the cell wall. These can be substances required for body biochemistry or unwanted byproducts. Alkali cations function as the carriers for molecules across cell membranes.

LeChatelier's Equilibrium Perturbations: Concentration Change

Changing the concentration of a reactant or product that appears in the equilibrium expression temporarily knocks the system out of equilibrium. In an attempt to reestablish equilibrium, the system responds by increasing the rate of either the forward or reverse reaction. Increasing [Reactant] or decreasing [Product] will shift the system to the right, towards product formation. Decreasing [Reactant] or increasing [Product] will shift the system to the left, towards reactant formation. In either case, the system will eventually return to the original position of equilibrium dictated by the Keq of the system.

Biological Roles of Na+ & K+

Charge carriers in essential body electrolytes required to maintain homeostasis. Required for nerve synapses, Na+ is the main cation for extracellular fluids, and K+ is the main cation for intracellular fluids.

Coordination Complex Properties

Coordination complexes are molecules that consist of a transition metal ion bonded to a polar molecule via coordinate covalent bonds (H3B-NH3 is a coordination complex) The Lewis Base/Acid character of the electron donor (polar) and acceptor (transition metal) molecules involved in coordinate covalent bonding means that the formation of a coordination complex constitutes a Lewis Acid-Base reaction.

Coulomb's Law

Coulomb's Law tells us that like charges repel (in which case F is positive) and opposite charges attract (in which case F is negative) It also reveals that the force, whether positive or negative, is proportional to the charges on the species, and inversely proportional to the square of the distance between them. What this means is that if the distance between two charges is doubled, then then force between them drops to 1/4 its previous value.

Determining Hybridization

Each pair of electrons must be housed in an electronic orbital, be it s, p, d, or f. Orbitals are always used up in the order: s p p p d d d d d etc. Determining the number of distinct electron groups allows you to determine hybridization state. Starting with the single s and moving right, simply circle one letter for each individual electron group. Then, sum them up. The sum of the exponents tells how many orbitals of a type are used. EX: Methane has four single covalent bonds surrounding it's central carbon. Each of these bonds represents one electron group which needs its own orbital to house it. -> s + p + p + p = sp3 --> sp3 hybridized carbon atom

Electron Affinity

Electron affinity of an atom or ion is the amount of energy involved in adding a single electron to an isolated gas phase atom represented by: X (g) + e- --> X- (g)

Repulsive IMFs

Electron cloud repulsion (which is the driving force of the VSEPR theory) prevents atoms, which are mostly empty space, from passing through eachother. This is why you can stand on the floor without falling through it. In orgo/biochem, electron cloud repulsion is known as steric hinderance, but for now it is sufficient to know that electron clouds repel one another- a type of repulsive IMF

Electronegativity Trend

Electronegativity generally increases moving left to right across rows and up columns. The most strongly electronegative atoms are in the top right and the weakly electronegative atoms are in the bottom left. This trend excludes the noble gases.

Metalloenzymes

Enzymes are simply biological catalysts. Metalloenzymes are an important class of enzymes characterized by the presence of a metal ion that is an essential participant in catalyzed reactions; two examples are: (1) Carboxypeptidase A: Catalyzes the hydrolysis of the C-terminal residues in peptide chains with the participation of Zn+2. (2) Carbonic anhydrase: zinc metalloenzyme found in plants, animals, and microorganisms. Catalyzes the reversible hydration of CO2.

Biological Roles Co+2

Essential component of vitamin B12

Other Ionization Energies

Following the first ionization, additional electrons can be removed as long as the atom has them. Each electron that is removed requires a different ionization energy. Second Ionization Energy, for example, is the energy required for: X+ (g) + energy --> X+2 (g) + e- Second/third/etc IEs do not follow a defined trend like the First IE does, however, it is a fact that each subsequent IE will always be greater than the previous one. This means: 4th IE> 3rd IE> 2nd IE> 1st IE

Basicity Trend

Follows the opposite trend to that of acidity. Basicity increases moving right to left or up a column on the periodic table. Noble gases are neither acidic or basic. The most basic elements are in the top left The least basic elements are in the bottom right

Acidity Trend

Follows the opposite trend to that of basicity. Acidity increases moving from left to right or down a column on the periodic table. Noble gases are neither acidic or basic. The most acidic elements are in the bottom right The least acidic elements are in the top left

Negative and Positive EA Values

For most elements, process is exergonic (outputs energy). EX: F (g) + e- --> F- (g) + 322 kJ EA = -322 kJ/mol Since Fluorine wants to pick up an electron to fill out its 2p energy sub level, energy is released and the process is favorable, resulting in a negative value for EA. A negative EA value means that the element readily accepts the e-. Some elements, such as the whole Beryllium family and the Noble gases will not readily accept an electron. In these cases the process is endergonic (requires energy). EX: Be (g) + e- + 241 kJ --> Be- (g) EA = +241 kJ/mol Since Beryllium wants to lose electrons to become stable, it is strongly opposed to taking on any new e-, meaning that energy must be absorbed to make it happen. Since energy must be input, the process is unfavorable, resulting in a positive value for EA. A positive value for EA means that the element will not readily accept e-.

Volume of Cubic Close Packing Unit Cells

For simple cubic close packing cells, with its spheres in each of its eight corners touching their immediate neighbors, the length (L_ccp) of one edge of the cubic cell is equal to twice the radius of the sphere. The volume (V_ccp) of one cell is equal to side length cubed.

Heme Proteins

Heme proteins have several biological funcitons: -They are involved in electron transfer reactions (cytochrome c, cytochrome c oxidase, cytochrome P450). - They act as oxygen carriers (myoglobin, hemoglobin) - Many catalyze biochemical reactions (peroxidases) All heme proteins are catalyzed by an active group, the heme, which is embedded in a protein matrix that consists of folded, linked amino acid chains. The heme is a macrocyclic porphyrin ring that contains iron as its central metal and whose chemistry is influenced by ring substituents that can differ from one type of heme protein to another. The iron can be 4, 5, or 6 coordinate and is always coordinated to 4 nitrogens in the porphyrin ring. There are two additional coordination sites, above and below the plane of the ring. In hemoglobin, the 5th ligand coordination occurs with the nitrogen of the side chain of a histidine residue. The 6th ligand can be oxygen or carbon monoxide.

First Ionization Energy Trend

IE increases moving up a column or to the right (with decreasing atomic size) Elements with the highest IEs are in the top right Elements with the lowest IEs are in the bottom left

Intermolecular Forces (IMFs)

IMFs are the forces that exist between two or more molecules. All IMFs are forms of the electromagnetic force and involve the interaction between charges. Since all molecules experience some degree of IMFs (a form of electromagnetic force), this implies that even neutral molecules have some form of uneven charge distribution, because when we are talking about electromagnetic force (which IMFs are a form of) we are talking about charge-charge interactions. A pair of charges may only interact in two ways, so we expect IMFs to come in two different types, cohesive (attractive forces) and repulsive (repelling forces). The strength of these interactions is embodied in Coulomb's Law, which quantifies electrostatic force.

Nonpolar Covalent Bonds

If the electronegativities of the bonding atoms are identical or nearly identical (not different by more than ~0.4 units), then the bonding electrons will be shared equally, spending about the same amount of time around each atom. Most nonpolar bonds exist between atoms of the same element (C-C, H-H, etc.) but bonds between atoms with nearly identical electronegativities are also classified as nonpolar (C-H, B-Si, etc.)

Inverse Polarization of p-n Junctions

If the p-zone is connected to the negative pole of a battery, and the n-zone is connected to the positive pole, then the electrons in the n-zone are attracted to the positive pole and the positive holes in the p-zone are attracted to the negative pole. This results in a greater region of depleted charge within the junction as the charges are clustered around the charged power terminals. This is opposite to the normal direction of displacement of the holes and electrons at the junction. Reverse displacement means that the junction offers a resistance to the flow of current. When no current is able to flow through the system in this way, the p-n junction is said to be inversely polarized.

Direct Polarization of p-n Junctions

If the p-zone is connected to the positive pole of a battery, and the n-zone is connected to the negative pole, then the electrons in the n-zone are attracted to the positive pole and the positive holes in the p-zone are attracted to the negative pole. Because the regions on either side of the junction are now oppositely charged, the flow of electrons moves through the junction and is in the normal direction of displacement. Normal displacement means that the current in the junction is able to flow freely. When current is able to flow freely through the system in this way, the p-n junction is said to be directly polarized.

T, O, and Triangular Holes

If there are N atoms in a crystal, there are N O-holes and 2N T-holes. The O-holes are larger and can accommodate a sphere of of up to 0.41x radius of the largest sphere without causing structural distortion. T-holes can only accommodate about 0.23x radius of the largest sphere. Tetrahedral holes (T-Holes) are created by the packing of four spheres, three in a triangular formation on the first layer, and one above. These spheres create a rectangular tetrahedron about a void, which is how they get their name Octahedral holes (O-Holes) are created by the packing of six spheres, three in a triangular formation on the first layer, and three above in the second layer offset by 60deg with respect to the first layer. The void in the center is an O-hole Triangular holes can also be created, between three adjacent spheres in the same layer For spheres of equal diameter, the size of the holes is: triangular < tetrahedral < octahedral

Band Theory

In Band Theory, the metal is thought of as a giant molecule in which delocalized molecular orbitals cover the entire structure (since the atoms lie in a 3D array and bonding spreads over the entire crystal). A crystal made up of N atoms with overlapping atomic orbitals will have a total of N Molecular Orbitals (MOs), with N distinct energy levels. These bonding and antibonding orbitals are so closely spaced in terms of energy that they can be referred to as a "band" and are virtually continuous. Two special bands, the valence band and the conduction band, give us more information about conductivity.

Dipole IMFs

In a fluid medium, electrostatic forces cause polar molecules to align their charged region near the oppositely charged regions of their neighbor. The interactions between these charged regions are called dipole forces. Dipole forces are weaker than ionic forces, but they still have significant effect on the physical properties of a compound. In general, the strength of a dipole-dipole interaction is only about 1% the strength of ion-ion interactions, with the important exception of hydrogen bonding interactions.

Lewis Base Ligands

In coordination chemistry the Lewis Base is called the ligand or the chelator. Some common base ligands include: -ammonia, water, alcohols, halide ions, cyanide ions, carbon monoxide, and more other nitrogenous molecules Some of these ligands (ex: cyanide, carbon monoxide) form strong coordinate covalent bonds, while others (ex: water, halide ions) form weak coordinate covalent bonds, resulting in weakly bonded coordination complexes that can easily fall apart.

Volume of Face Centered Unit Cells

In face centered cells, the spheres touch along the face diagonal, the length of which is given by 4x radius. The length (L_fcc) of the edge of the face centered cell and its volume (V_fcc) are related to the length of the face diagonal.

Consequences of Lattice Energy on Solubility

In general, compounds that contain ions with widely different radii are usually soluble in water. Some examples of this are the sulfates of the alkaline earth elements. The sulfate ion is quite large. This means that the solubility of alkaline earth metal sulfates decreases as the alkaline earth metal component increases in size, since the difference is radii gets smaller and smaller moving from magnesium to barium. Because barium sulfates are relatively insoluble, the barium ion is used in the gravimetric determination (which involves isolation of an ion in a solution through precipitation) of sulfate ions In contrast to the sulfates, the solubilities of earth metal hydroxides show and increase when moving from magnesium to barium hydroxide. Since the hydroxide ion is small, bigger metals create a larger radii difference and are more soluble.

Consequences of Lattice Energy on Thermal Stability

In general, large anions are stabilized by large cations, and in the same way, large cations are stabilized by large anions. Take for example the decomposition temperature of thermally unstable carbonate compounds: -Magnesium carbonate decomposes at 300 C -Barium carbonate does not compose before 800 C The difference is that the carbonate anion is pretty large, and so the small magnesium cation is not able to stabilize it as well as the larger barium cation, giving it a lower decomposition temp due to its lower stability

Close Packing of Spheres

In substances where the atoms are packed together as closely as possible, the structures represented by the spheres are called close packed structures. This type of geometry allows for the maximum number of neighbors with the minimum amount of wasted space. The structures of many solids can be described in terms of the regular stacking of spheres that represent the ions or atoms. Spheres in close packed structures have several layers. In the first layer, a single sphere is surrounded by six neighbors. In the second layer, the spheres are not placed directly above those on the first layer, but instead are placed to occupy the voids between adjacent spheres. This packing creates O, T, and triangular holes discussed elsewhere. The third layer may be arranged in either of two ways, leading to two possible structures. (1) Hexagonal Close Packing: The spheres of the third layer may lie directly above the spheres of the first layer. This packing of layers yields a lattice with a hexagonal unit cell (hence the name) (2) FCC Close Packing: The spheres of the third layer may be placed directly above the holes of the second layer. In this arrangement, the third layer lies over neither of the previous layers (the fourth layer would be directly above the first) and this repeats in an abc-abc fashion. This packing of layers yields a lattice with face centered cubic (FCC) unit cells.

Crystal Lattices

Ions in an ionic compound (also known as a salt) are arranged in a regular pattern called a crystal lattice. The term crystal simply refers to a solid arranged in this way. The energy of the electrostatic interactions within the crystal is called the lattice energy and is denoted as an italicized (U). The array of a crystal lattice is regular and repeats itself periodically. The smallest unit that repeats itself indefinitely in three dimensions is called a unit cell. An entire branch of chemistry, Crystallography, is dedicated to the study of unit cells.

Hierarchy of the Top 6 Electronegative Atoms

LOWER--- C < Br < Cl < N < O < F ---HIGHER The fact that F, O, N have the greatest electronegativity values has profound influence on the IMFs that exist between molecules containing them.

Dispersion IMFs

London dispersion forces are experienced by any atom or ion that has at least one electron. They arise from a momentary non-homogeneous distribution of the electrons of an atom. This happens because the electrons are flying around- even in the nuclei of a perfectly nonpolar molecule, it will happen that more electrons are on one side of the nucleus than the other. This electron imbalance means that the atom has a slightly positive and a slightly negative side, known as an instantaneous dipole. An instantaneous dipole in one molecule can induce an instantaneous dipole in another nearby molecule. By definition, dispersion force is the attraction between two adjacent oppositely charged instantaneous dipoles that just happen to have occurred at the same time. Since these instantaneous dipoles are very transient, dispersion forces are short ranged and weak. Individual dispersion forces are much weaker than either ionic or dipole forces. The cohesive contribution of dispersion forces in ionic or polar molecules is insignificant compared to ionic and dipole forces. However, in nonpolar molecules, dispersion forces are the sole cohesive force. The strength of dispersion forces generally increases as the number of electrons present increases, and thus the physical characteristics of large nonpolar molecules are actually influenced significantly by dispersion forces.

Polar Molecules & the Dipole Moment

Molecules with a dipole moment are called polar molecules, and they have an asymmetric distribution of electron density. This asymmetry results in one end of the molecule having a partial negative charge, while the other end has a partial positive charge, even though the overall charge of the molecule remains neutral at 0.

Molecular Shapes

Molecules with: (a) Linear geometry (2 e- groups) may have: -0 nonbonding groups: Linear shape (b) Trigonal Planar geometry (3 e- groups) may have: -0 nonbonding groups: Trigonal planar shape -1 nonbonding group: Bent shape (c) Tetrahedral geometry (4 e- groups) may have: -0 nonbonding groups: Tetrahedral shape -1 nonbonding group: Trigonal pyramid shape -2 nonbonding groups: Bent shape (d) Trigonal bipyramidal geometry (5 e- groups) may have: -0 nonbonding groups: Trigonal bypyramidal shape -1 nonbonding group: Seesaw shape -2 nonbonding groups: T shape (e) Octahedral geometry (6 e- groups) may have: -0 nonbonding groups: Octahedral shape -1 nonbonding group: Square pyramid shape -2 nonbonding groups: Square planar shape

Unit Cells of Crystal Systems

Most of the 7 basic crystal systems can be divided further as there are some distinct unit cell types A simple cubic primitive unit cell (P) contains atoms only at the corners of the cell A body centered unit cell (bcc or I) contains atoms at the corners and one at the center of the unit cell A face centered unit cell (fcc or F) contains atoms at its corners and one in the center of each of the six cell faces

N-Type Extrinsic Semiconductors

N-Type semiconductors get their name from their negative electron charge carriers. They are formed when the atom used for doping (as an impurity) possesses more external electrons than the original parent atom did. After the electrons necessary for covalent bonds to the neighboring semiconductor atoms have been accounted for, the remaining electron(s) are available as donor electrons. The donated electrons of each donor atom (the dopant impurities) interact with each other to form a Donor Band (this Donor Band narrows as donor atoms become more spread apart). This Donor Band is close to the Conduction Band, and so thermal excitation will be sufficient to promote the donor electrons to the Conduction band, creating a semiconductor.

Metallic Bonding Structure & Properties

One model of metallic structure is that the lattice of positive metallic ions exists in a sea of electrons that hold the ions tightly together. Atoms in a crystal structure can be easily displaced in planes with respect to each other, explaining the malleability (ability to be hammered into shapes) and ductility (ability to be drawn into sheets and wires) of metal This plane displacement in the sea of electrons provides a constant shield between the positive ions and does not allow for the development of strong repulsive forces

Cohesive IMFs

Oppositely charged molecules with greater magnitude of charge (or separation of charge in the case of neutral molecules) will experience greater intermolecular attraction, since, according to Coulomb's Law, force is proportional to charge (assuming internuclear distance is the same). There is great diversity in the strength of attractive IMFs, which shows in composition: - Hard, high melting point solids (geological minerals, rocks) are a consequence of very strong IMFs - Tenuous gases (molecular oxygen or nitrogen) are the result of very feeble IMFs - Liquids experience IMFs between these two

Biological Roles of Mn, Cr, Ni and Mo

Other essential trance elements

Oxidation Numbers in Redox Reactions

Oxidation involves an increase in oxidation state (oxidation number becomes more positive, element is oxidized) Reduction involves a decrease in oxidation state (oxidation number becomes more negative, element is reduced) In a balanced chemical equation, the oxidation and reduction must balance exactly.

Oxidation State/Number

Oxidation state shows the total number of electrons which have been removed from an element (a positive oxidation state) or added to an element (a negative oxidation state) to get to its present state.

P-Type Extrinsic Semiconductors

P-Type semiconductors get their name from their positive hole charge carriers. They are formed when the atom used for doping (as an impurity) possesses fewer external electrons than the original parent atom did. These dopant atoms form a very narrow, empty Acceptor Band. At T = 0 K, the Acceptor Band remains empty, but at higher temps, thermal excitation allows excited electrons to be promoted into the Acceptor Band, creating holes (one per dopant atom) in the Valence Band. This allows other electrons in the valence band to become mobile. An electron from a neighboring dopant atom will then drop into the positive hole, creating a vacancy for the next semiconductor atom to fill with an electron. The result is a cascade effect in which an electron from each row of atoms moves one place towards the neighboring atom. This cascade effect is what allows conductivity without the involvement of the Conduction Band. This means that electrons are mobile in both the Valence Band and the Acceptor Band. The process of movement in the Valence Band can also be thought of as a positive hole moving across a row of atoms in the direction opposite the flow of electrons. This mobility leads to extra hole energy levels, thereby creating a semiconductor.

Polar Covalent Bonds

Polar molecules arise from internal polar covalent bonds. A polar covalent bond forms between two elements with differing electronegativities and is uneven. This unevenness is because the bond is being pulled by both atoms, and the one with the higher electronegativity will be able to pull more strongly (since electronegativity is a measure of an element's ability to reel in electrons). Since the bond itself has a negative charge (it is made of electrons), this means that the more electronegative atom acquires a partial negative charge, leaving the more electropositive atom with a partial positive charge. Conceptually, polar bonds lie somewhere between the evenly shared electrons in a nonpolar covalent bond and the complete polarization that forms ionic bonds.

The VSEPR Theory

Predicts the shape of simple molecules and has one rule: *Electron pairs, whether bonding or nonbonding, attempt to move as far apart as possible* The atom's geometric identity is based solely on the number of electron groups involved. Once this geometry has been pinpointed, overall shape can be determined. When counting the number of electron groups, bonding and nonbonding electron pairs are counted as one electron group per pair (as expected), and double/triple bonds only count as one electron group.

Biological Roles of Fe+3 & Fe+2

Present in heme proteins, such as myoglobin (O2 storage in muscle), hemoglobin (O2 carrier in blood) and in the cytochromes (e- transfer in mitochondrial respiration chain)

Polar Molecules

Simply having polar bonds does not necessarily make a compound polar. The overall structure of the molecule must also be considered. In highly symmetric molecules, polar bonds may be oriented in such a way that the dipole moments cancel out. These dipole moments are symbolized by an arrow pointing from the atom with the partial positive charge to the atom with the partial negative charge. If a molecule is polar, it behaves like a very weak ion

Electronegativity

The ability of an atom to attract electron density from other atoms to which it is bonded (polarize bonding e- towards itself). Different from Electron Affinity as it deals strictly with bonded atoms. Highly electronegative atoms pull electrons more strongly and usually get them, resulting in increased electron density Weakly electronegative atoms are usually the victims of this pulling and have decreased electron density

Band Charachteristics

The actual width of a band depends on the degree of overlap of the atomic orbitals of neighboring atoms- the greater the overlap, the wider the band is. Because of all this overlap, a band is a near continuum of a finite number of energy levels. A band constructed from the overlap of s orbitals is known as an s band. P and D bands can be constructed from the overlap of appropriate orbitals as well

Blue Copper Proteins

The blue copper proteins (azurin, plastocyanin, and stellacyanin) are involved in electron transport and copper storage functions. Their active group consists of a copper ion coordinated to amino acid residues which results in a distorted tetrahedral geometry.

Conduction Band

The conduction band is made up of high energy orbitals that are generally empty. It is the distance between the Valence Band and the Conduction Band (the Energy Gap) that determines whether electrons may jump from the Valence Band to the Conduction Band. If electrons have jumped to the Conduction Band from the Valence band, the material is able to conduct electricity

Coordination Numbers

The coordination number (CN) of an atom or ion is the number of its closest neighbors within the lattice, typically between 1 and 6. The CN depends on the size of the central atom, as well as the sizes of the surrounding atoms. The smaller the surrounding atoms, and the larger the central atom, the higher the possible value for CN is. These coordination numbers are associated with specific geometries, for example: -CN = 2 defines linear geometry -CN = 3 defines a trigonal pyramidal or trigonal geometry -CN = 4 defines a tetrahedral or planar (rare) geometry

7 Basic Structures of Ionic Solids

The crystal structure of many ionic solids is usually described in terms of the systematic filling of O and/or T-holes in a close backed structure of ions (usually anions) by smaller ions (usually cations). Ionic substances tend to adopt one of seven basic structures. Most can be seen as lattices in which the anions (usually the largest ions) stack together in cubic patterns with cations (usually the smallest ions) occupying the O or T-holes The 7 basic structures are: (1) Rock Salt (or Halite) Structure INC: NaCl, AgCl, AgBr, KBr, LiCl, RbI, TiO, MgO, CaO, etc. In NaCl, the structure is based on a cubic array of bulky Cl- anions, with the Na+ cations occupying all of the O-holes (2) Zincblende (or Sphalerite) Structure INC: ZnS, CdS, HgS, CuCl. etc. In ZnS, the structure is based on a cubic array of Zn+2 cations, with the S-2 anions occupying half of the T-holes (3) Fluorite Structure INC: CaF2, BaCl2, PbO2, HgF2, etc. In CaF2, the structure is based on a cubic array of Ca+2 cations, with F- anions occupying all of the T-holes (4) Anti-Fluorite Structure INC: K2O, K2S, Li2O, Na2O, etc. In K2O, the structure is based on a cubic array of O-2 anions, with K+ cations occupying all of the T-holes. This is simply the inverse of the Fluorite structure, with the cation and anion positions reversed. (5) Wurzite Structure INC: ZnS, ZnO, MnS, BeO, etc. In ZnS (another polymorph of zinc sulfide as seen in (2)) the structure is based on a hexagonal array of Zn+2 cations, with S-2 anions occupying half of the T-holes (6) Nickel-Arsenide Structure INC: NiAs, NiS, CoS, FeS, etc. In NiAs, the structure is based on a hexagonal array of As-3 anions, with the Ni+3 cations occupying all of the O-holes (7) Rutile Structure INC: TiO2, MnO2, MgF2, SnO2, NiF2, etc. In TiO2, the structure is based on a hexagonal array of O-2 anions, with the Ti+4 cations occupying half of the O-holes

Relative Electronegativity Values*

The electronegativity scale is ~1-4. It is unitless and somewhat arbitrary, but allows values to be compared to one another. These relative values in a molecule play an important role in allowing us to make qualitative predictions regarding atomic interactions and bonding If relative values are equal or similar, the bond is nonpolar. If they are not, the bond is polar or ionic, depending on the elements and the exact differences

Ionic IMFs

The electrostatic attraction between two ions is simply referred to as an ionic bond. As used here, the word bond is somewhat misleading as there are no electrons shared in an ionic bond, unlike a covalent bond. Ionic bonds are the strongest type of IMF, and the actual force holding them together can be calculated using Coulomb's Law, but the main point to consider is that Coulomb's Law states that if we assume the distances between all pairs of ions are identical, then the ionic force of electrical attraction is proportional to the magnitudes of the charges of the ions. This makes it clear that the strength of the ionic bond is greater between highly charged ions than between singly charged ions. This is a critical point because many chemical and physical processes (such as melting, boiling, solubility) depend on how easily ions can be separated.

First Ionization Energy

The energy that is required to remove a single electron from an isolated gas phase atom X (g) + energy --> X+ (g) + e- IE values depend on ionic size- the smaller the atom, the greater the IE. Small elements have high 1st IE (since e- are close to nucleus and tightly held), while larger elements have low 1st IE (e- further and more loosely held)

Equilibrium Constants

The equilibrium constant, K, encodes the relative amounts of reactants and products when the system is at equilibrium, since there is no way to determine this just by looking at the reaction. Its actual numerical value for a given equilibrium can be determined by plugging equilibrium concentrations into the correct equilibrium expression. These equilibrium constants are almost always followed by a subscript. These exist mainly for tracking, and they indicate what sort of chemical process is occurring. These subscripts do not change any of the rules or regulations that equilibrium constants are bound by, so don't let them confuse you. If they prove to be intimidating, just cross them out and treat it as a regular Keq. Some are: -Keq: generic equilibrium constant -Ka: for acid equilibrium -Kb: for base equilibrium -Kp: for gaseous equilibrium -Ksp: for dissolution equilibrium of ionic substances

Born-Haber Cycle Example

The formation of sodium fluoride from its constitutents RXN: Na (s) + 1/2 F2 (g) --> NaF (s) The energetic factors associated with each step are obtained by breaking the reaction down into steps. *H = deltaH (change in H) = change in enthalpy (1) Sublimation of solid sodium Na(s) --> Na (g) *Hsublimation = +109 kJ/mol (2) Ionization of gaseous sodium atoms Na (g) --> Na+ (g) + e- *Hionization = +496 kJ/mol (3) Dissociation of fluoride molecules F2 (g) --> 2 F (g) *Hdissociation = +154 kJ/mol For one mole of F atoms, enthalpy = +77 kJ/mol (4)Formation of fluoride ions (electron affinity) F (g) + e- --> F- (g) *Hionization = -328 kJ/mol (5) Formation of sodium fluoride from gaseous sodium and fluoride ions Na+ (g) + F- (g) --> NaF (s) *Hlattice = -923 kJ/mol The sum of the five reaction processes represents the global reaction, and the sum of the five individual enthalpy values is equal to the total energy value: Na (s) + 1/2 F2 (g) --> NaF (s) ; *Htotal = -569 kJ/mol

Semiconductors vs. Insulators

The size of the energy gap between the Valence Band and the Conduction Band is the biggest difference between an insulator and a semiconductor Because both insulators and semiconductors can be made to conduct electricity (one more readily than the other) if sufficient energy is provided to cross the energy gap, the distinction between them may be considered artificial and is sometimes ignored because it complicates things: EX: Silicon has an energy gap of 25 kcal/mol, and so it is sometimes considered an insulator, and sometimes considered a semiconductor in its pure state. When mixed with certain impurity atoms, a silicon crystal is considered a semiconductor. So the difference is kind of arbitrary

Valence Band

The valence band contains any freely moving valence electrons. It may be partially or completely filled. When a valence band is not fully occupied, valence electrons can move in many energy states. In some cases, electrons from the valence band are able to jump into the conduction band, allowing the material to conduct electricity.

ADD IMG The 7 Crystal Systems

There are seven types of crystalline structures, each of which possesses a characteristic unit cell that repeats itself to yield a solid structure without voids. The size and geometry of a unit cell is denoted by: -lengths (a,b,c) of three axes, also called edges -angles (alpha A , beta B , gamma Y) three occur at -intersections (b&c, a&c, a&b) between pairs of axes The Seven Crystal Systems are: (1) Cubic EX: NaCl Edges: a = b = c Angles: A = B = Y = 90deg (2) Tetragonal EX: Hg(CN)2 Edges: a = b =/= c Angles: A = B = Y = 90deg (3) Hexagonal EX: PbI2 Edges: a = b = c =/= d Angles: A = B = Y = 120deg *Edge d perpendicular to plane described by edges abc (4) Rhombohedral EX: NaNO3 Edges: a = b = c Angles: A = B = Y =/= 90deg (5) Orthorhombic EX: K2CrO4 Edges: a =/= b =/= c Angles: A = B = Y = 90deg (6) Monoclinic EX: K3Fe(CN)6 Edges: a =/= b =/= c Angles: A = Y = 90deg ; B =/= 90deg (7) Triclinic EX: CuSO4 * 5 H2O Edges: a =/= b =/= c Angles: A =/= B =/= Y

Hydrogen Bonding Interactions

These are a special type of dipole-dipole interaction that is the exception to the 1% strength rule. Any molecules that contain an H-O, H-F, or H-N bond experience unusually strong dipole-dipole interactions that range from 3%-5% of the magnitude of the ion attraction in NaCl. GRE Definition of a Hydrogen Bond: A hydrogen bond is a strong dipole-dipole interaction involving a hydrogen atom covalently bonded to a fluorine, oxygen, or nitrogen atom. that is attracted to the partial negative charge of another (separate) atom of fluorine, oxygen, or nitrogen.

Covalent Bonds

To an atom, covalent bonds are nothing but a means to and end of closed shell stability. Atoms with similar electronegativities cannot give or take electrons in order to meet their closed shell requirements. This leads to a compromise, the covalent bond. Typically, each atom will contribute one electron (for a total of 2) to form the covalent bond between them.

Lattice Energies & Coulombic Contributions; Born-Lande

To calculate the lattice energy in an ionic solid, we must account for attractions and repulsions between ions involved. It is important to note that the Born-Lande equation only accounts for the attractive component of the total potential energy of the solid. The lattice energy, which can also be defined as the energy released when one mole of the free gaseous ions come together from infinite interionic separation to make up a crystal can be calculated from the Born-Lande equation. The Born-Lande equation tells us that for a given crystal type (value of M) the lattice enthalpy increases with increasing ionic charge and decreasing ionic radii. The variables in the Born-Lande equation are: N_A: avogadro's number, 6.022 * 10^23 mol^-1 z+ & z-: charges of the +/- ions E_0: vacuum permittivity constant, 9.95 * 10^-12 C^2J^-1m^-1 e: electron charge, 1.602 * 10^-19 C (r+ + r-): equilibrium distance between ions M: Madelung constant n: Born's exponent The value of n (Born's exponent) is usually between 7 and 10 and is related to ion size. M, the Madelung constant, is the sum of an infinite series, and it represent the effect on the ion made by its neighboring atoms. This reflects relative positions of ions within the crystal, thus M is dependent on crystal geometry. Some common M values are: -cesium chloride (1.763) - fluorite (2.519) -rock salt (1.748) - rutile (2.4008) -sphalerite (1.638) -wurzite (1.641)

Lattice Energies & Coulombic Contributions; Born-Mayer

To calculate the lattice energy in an ionic solid, we must account for attractions and repulsions between ions involved. Unlike the Born-Lande equation, the Born-Mayer equation takes both the attractive and repulsive components of the total potential energy of the solid into account. Like the Born-Lande equation, the Born-Mayer Equation tells us that for a given crystal type (value of M) the lattice enthalpy increases with increasing ionic charge and decreasing ionic radii. The two new variables are: d*: a constant estimated from measures of compressibility, usually about 0.345 angstrom d: the distance to the closest ion Most variables are identical to those in the Born-Lande: N_A: avogadro's number, 6.022 * 10^23 mol^-1 z+ & z-: charges of the +/- ions E_0: vacuum permittivity constant, 9.95 * 10^-12 C^2J^-1m^-1 e: electron charge, 1.602 * 10^-19 C (r+ + r-): equilibrium distance between ions M: Madelung constant M, the Madelung constant, is the sum of an infinite series, and it represent the effect on the ion made by its neighboring atoms. This reflects relative positions of ions within the crystal, thus M is dependent on crystal geometry. Some common M values are: -cesium chloride (1.763) - fluorite (2.519) -rock salt (1.748) - rutile (2.4008) -sphalerite (1.638) -wurzite (1.641)

LeChatelier's Principle

We know that chemical systems in dynamic equilibrium will remain in equilibrium indefinitely unless they are acted upon by an outside influence. LeChatelier discovered that there are three perturbations that can, in principle, disturb a chemical equilibrium. They are as follows: (1) Change in Concentration (2) Change in Pressure (3) Change in Temperature

Coordinate Covalent Bonds

When a covalent bond forms between a transition metal ion (such as Fe+3 Or Cr+3) and a polar molecule (such as H2O or NH3) it is called a coordinate covalent bond. These bonds are what hold together coordination complexes. The bond itself is formed by a pair of electrons donated by the polar molecule. This molecule is the donor molecule, and is thus a Lewis Base, while the molecule that accepts the pair (the acceptor molecule) is a Lewis Acid. The Lewis Base/Acid character of these molecules means that the formation of a coordination complex constitutes a Lewis Acid-Base reaction.

Lattice Energies & the Born-Haber Cycle

When an ionic solid is formed from mutually attractive cations and anions, the overall energy of the solid is lower than that of the free ions. The difference in energy between the two forms is called the cohesive energy. The lattice energy is the sum of the energies of interaction of the ions in the crystal. This cannot be measured directly, and is the result of two main contributions- the Coulombic interactions between ions and the van der Waals repulsive energy. The Coulombic interaction can be evaluated theoretically using compressibility data. It is known as the Mandelung constant. To get around the impossibility of directly measuring lattice energy, experimental thermochemical data can be used in conjunction with the Born-Haber cycle. The Born-Haber cycle is a calculation of the total energy of a crystal, determined by considering each of its formation steps and including a lattice energy contribution

Polar Covalent Bonds

When atoms have a higher electronegativity difference (~0.5-1.5 units) the bonding electrons will spend more time around the more electronegative atom. Because of this, the more electronegative atom develops a partial negative charge and the less electronegative atom develops a partial positive charge. Generally, the greater the electronegativity difference between bonding atoms, the greater the polarization (unevenness) of the bonding electrons. This polarization results in polar bonds, which are fatter at one end than the other. (ex: C-F, O-H, Li-C, S-Cl) Polar covalent bonds are a "middle ground" between the equal sharing of nonpolar bonds and the complete electron transfer observed in ionic compounds.

Atomic Size Comparison of Isoelectronic Species

When comparing two isoelectronic species (ie, two atoms or ions with the same number of electrons) the one with the greater atomic number will have more protons to pull the electrons in closer. This means the one with the greater atomic number will have the smaller atomic radius. EX: F- is smaller (z=9) than O-2 (z=8), both have 10 total electrons.

Metallic Covalent Bonding & Valence Shells

When examining the electron configuration of common metals, it can be seen that they always have more valence orbitals than valence electrons to fill them. This is part of the reason that metals have such high conductivity. EX: Li (1s^2 2s^1 2p^0) has 3 electrons, and the first two are in the full 1s orbital. This means it has only one electron in all of energy level 2. This remaining single electron is shared between the 2s and 2p orbitals, and it relatively free to move around the crystal lattice structure.

Covalent vs. Ionic Compounds

When the electronegativities of a compound's constituent elements differ by no more than about 1.5 units (ex: compounds made of exclusively nonmetal elements) then electrons will be shared, and the atoms will be covalently linked. (ex: CO2) When the electronegativities of a compound's constituent elements differs by more than 1.5 units (ex: compounds made of a metal and a nonmetal) the compound is ionic For the most part, this means that when an atom from the far right of the periodic table (in the highly electronegative area) combines with an atom from the far left (the weakly electronegative area) the compound is likely to be ionic Just about every other combination with be some type of covalent bond

Electron Loss/Gain Tendencies

Within the periodic table, elements to the left and center (metals) typically lose their valence electrons and become positively charged cations -H is considered a nonmetal, but its reactions are similar to those of metals. Elements to the right (nonmetals) tend to supplement the electrons they already have in order to gain electrons and become negatively charged anions Metalloid elements (right along the staircase delineation) can posses properties characteristic of metals or nonmetals This divergence in tendencies is the primary cause of the chemical and physical differences between metals and nonmetals

Biological Roles Zn+2

pH control, liver function, and DNA synthesis