SAT Chemistry

..in a neutral atom..

..the number of electrons is equal to the number of protons.

Covalent bonds

A chemical bond that involves the sharing of electron pairs between atoms. These electron pairs are known as shared pairs or bonding pairs. For many molecules, the sharing of electrons allows each atom to attain the equivalent of a full outer shell, corresponding to a stable electronic configuration.

Amu

A unit that can used to report the mass of an atom is the atomic mass unit (amu). One amu is approximately the same as 1.66 x 10^-24g.

Ion

An atom or molecule in which the total number of electrons is NOT equal to the total number of protons, giving the atom or molecule a net positive or negative electrical charge.

Protons

Carries a single positive charge and has a mass of approximately one atomic mass unit or amu.

Neutrons

Carries no charge and have a mass only slightly larger than that of protons. Mass number determines its mass. (Neutron = mass number minus atomic number)

Orbitals

Characteristic regions outside the nucleus where electrons exists.

Valence electrons

Electrons of an atom that are in its outer energy shell or that are available for bonding.

!

Elements in the same group (same column) have the same electronic configuration in their valence, or outermost shell.

Sp3 hybridization

Formed by mixing an s and all three of the p orbitals. These hybrid orbitals point towards the four corners of a tetrahedron. *tetrahedral*

Nucleus

Formed by the protons and neutrons in an atom. It's the core of the atom.

Halogens

Group 17 elements. They are highly reactive nonmetals with 7 valence electrons. They have high electronegativity and are particularly reactive towards alkali metals and alkaline earths, which "want" to donate electrons to them to form stable ionic crystals.

Noble/Inert Gases

Group 18 elements. They are fairly nonreactive because they have a complete valence shell, which is an energetically favored arrangement. They thus have high ionization energies. They have low boiling points and are all gases at room temperature

Periods

Horizontal rows.

Isotopes

Isotopes are variants of a particular chemical element which differ in neutron number, although all isotopes of a given element have the same number of protons in each atom.

VSEPR theory

It states that the three-dimensional arrangement of atoms surrounding a central atom is determined by the repulsions between the bonding and nonbonding electron pairs in the valence shell of the central atom. The electron pairs arrange themselves as far apart as possible, thereby minimizing repulsion.

Sp2 hybridization

Mixing an s and two p orbitals. Three orbitals are mixed to form three hybrid orbitals. Bonds involved are 120degrees apart from each other. *trigonal planar*

Sp hybridization

Mixture of an s and a p orbital. 180degrees bond angle. *linear*

Anion

Negatively charged atom due to GAIN of electrons.

Types of Covalent Bonds

Polar covalent bonds, Nonpolar covalent bonds and Coordinate covalent bonds.

Cation

Positively charged atom due to LOSS of electrons.

Subatomic particles

Protons, Neutrons, and Electrons.

Electrons

Symbol e− or β−, with a negative elementary electric charge. Has a very small mass, approximately 1/1837 the mass of a proton or neutron.

Transition Elements/Metals

The 38 elements in groups 3 through 12 of the periodic table. As with all metals, the transition elements are both ductile and malleable, and conduct electricity and heat. The interesting thing about transition metals is that their valence electrons, or the electrons they use to combine with other elements, are present in more than one shell. This is the reason why they often exhibit several common oxidation states.

Bond Length

The average distance between the two nuclei of the atoms involved in bond. As the number of shared electron pairs increases, the two atoms are pulled closer together, leading to decrease in bond length. Thus, for a given pair of atoms, a triple bond is SHORTER than a double bond, which is in turns shorter than a single bond.

Lewis structure

The chemical symbol of an element surrounded by dots, each representing one of the s and/or p valence electrons of the atom.

Valence electrons

The electrons farthest from the nucleus. The farthest the valence electrons are from the nucleus, the weaker the attractive force of the positively charged nucleus and the more likely the valence electrons are to be influenced by other atoms.

Electron Affinity

The energy released when an electron is added to a gaseous atom, and it represents the ease with which the atom can accept an electron. The stronger the attractive pull of the nucleus for the electrons, the greater the electron affinity.

Ionization Energy

The energy required to completely remove an electron from an atom or ion. The more tightly bound an electron is to the nucleus, the more difficult it will be to remove, and the higher the ionization energy will be.

Bond Energy

The energy required to separate two bonded atoms. The strength of a bond increases as the number of shared electron pairs increases.

Orbital hybridization

The linear combination of atomic orbitals in a molecule to form hybrid orbitals.

Atomic weight/ Relative atomic mass

The mass in grams of one mole (mol) of atoms. A mole corresponds to about 6.022 x 10^23. (Avogadro's number). The atomic weight of an element is also found in the periodic table, as the number appearing below the symbol for the element.

Electronegativity

The measure of the attraction an atom has for electrons in a chemical bond. The greater the electronegativity of an atom, the greater its attraction for bonding electrons.

Mole (mol)

The mole is a unit of measurement for amount of substance. It is defined as the amount of any chemical substance that contains as many elementary entities, e.g., atoms, molecules, ions, or electrons, as there are atoms in 12 grams of pure carbon-12 (12C), the isotope of carbon with relative atomic mass 12 by definition.

Atomic number (Z)

The number of protons found in an atom of an element. All atoms of a given element have the same atomic number; in words, the number of protons an atom has defines what kind of element is it. The atomic number of an element can be found in the periodic table as an integer above the the symbol for the element.

Bond order

The number of shared electron pairs between two atoms.

Atom

The smallest building block of matter, representing the smallest unit of a chemical element.

Mass number

The total number of neutrons and protons in an atom.

Alkali metals

They are the elements in Group 1. They have only one loosely bound electron in their outermost shell, giving them the largest atomic radii of all the elements in their respective periods. Their high reactivity are determined by the fact that they have low ionization energies; thus, they easily lose their valence electron to form univalent cations (cation with +1 charge). They have low electronegativities and react with nonmetals, especially Halogens.

Alkaline Earth Metals

They are the elements in Group 2. They possess metallic properties. They have two electrons in their outer shell and thus have smaller radii than the alkali metals. The two electrons are not held very tightly by the nucleus, so they can be removed to form divalent cations. They have low electronegativity and electron affinity.

Pi bonds

This type of bond is formed by lateral or side wise overlap of the half filled atomic orbitals of the atoms participating in bonding. They consist of two charged clods above and below the plane of the atoms involved in bond formation.

Sigma bonds

This type of covalent bond is formed by the axial or end to end overlapping of half filled atomic orbitals of the atoms participating in bonding. The electron cloud formed as a result of axial overlap is cylindrically symmetrical about inter nuclear axis.

VSEPR

Valence Shell Electron-Pair Repulsion.

Groups

Vertical columns.

Triple bond

When three pairs of electrons are shared.

Double bond

When two pairs of electrons are shared.

Coordinate Covalent Bonds

a chemical bond between two atoms when one of the atoms shares a pair of electrons. An example of a coordinate bond is the relationship between hydrogen chloride and ammonia when the ammonium ions are formed by the transfer of a hydrogen ion to the pair of electrons in the ammonia molecule.

Ionic compounds

a chemical compound in which ions are held together in a structure by electrostatic forces termed ionic bonds. The positively charged ions are called cations and the negatively charged ions are called anions. These can be simple ions such as the sodium (Na+) and chloride (Cl−) in sodium chloride, or polyatomic species such as the carbonate ion (CO32−) in calcium carbonate. They have high boiling and melting points due to the strong electrostatic forces between the ions. They can conduct electricity in liquid and aqueous states, though not in the solid state.

Octet rule

a chemical rule of thumb that reflects observation that atoms of main-group elements tend to combine in such a way that each atom has eight electrons in its valence shell, giving it the same electronic configuration as a noble gas. The rule is especially applicable to carbon, nitrogen, oxygen, and the halogens, but also to metals such as sodium or magnesium.

Metals

a material (an element, compound, or alloy) that is typically hard, opaque, shiny, and has good electrical and thermal conductivity. Metals are generally malleable — that is, they can be hammered or pressed permanently out of shape without breaking or cracking — as well as fusible (able to be fused or melted) and ductile (able to be drawn out into a thin wire).

Dipole moment

a measure of the separation of positive and negative electrical charges in a system of electric charges, that is, a measure of the charge system's overall polarity.

Atomic radius

a measure of the size of its atoms, usually the mean or typical distance from the center of the nucleus to the boundary of the surrounding cloud of electrons. It decreases from left to right (column), and increases from up (period) on the periodic table.

Nonpolar Covalent Bonds

a type of bond that occurs when two atoms share a pair of electrons with each other. These shared electrons glue two or more atoms together to form a molecule. Occurs in diatomic molecules with the same atoms. Certain elements exists under normal conditions only as diatomic molecules: N2,O2,F2,Cl2,Br2,I2,H2. Their positions in the periodic table form an L-shape towards the top right, excluding the noble gases.

Ionic bonds

a type of chemical bond that involves the electrostatic attraction between oppositely charged ions. These ions represent atoms that have lost one or more electrons (known as cations) and atoms that have gained one or more electrons (known as anions). This transfer of electrons is known as electrovalence in contrast to covalence. In the simplest case, the cation is a metal atom and the anion is a nonmetal atom. In simpler words, an ionic bond is the transfer of electrons from a metal to a non-metal in order for both atoms to obtain a full valence shell.

Polar Covalent Bonds

a type of chemical bond where a pair of electrons is unequally shared between two atoms. The electrons are not equally shared because one atom spends more time with the electrons than the other atom. One atom has a stronger pull (electronegativity) than the other atom and attracts electrons. When electrons spend more time with one atom, it causes that atom to carry a partial negative charge. The atom that does not spend as much time with the electrons carries a partial positive charge.

Resonance Structure

are multiple Lewis Structures that collectively describe a single molecule. Resonance structures are necessary to describe molecules that contain fractional bonds and fractional charges.

Nonmetals

chemical elements that mostly lacks metallic attributes. Physically, they tend to be highly volatile (easily vaporised), have low elasticity, and are good insulators of heat and electricity; chemically, they tend to have high ionization energy and electronegativity values, and gain or share electrons when they react with other elements or compounds. Seventeen elements are generally classified as nonmetals; most are gases (hydrogen, helium, nitrogen, oxygen, fluorine, neon, chlorine, argon, krypton, xenon and radon); one is a liquid (bromine); and a few are solids (carbon, phosphorus, sulfur, selenium, and iodine).

Metalloids/ Semi metals

chemical elements with properties in between, or that are a mixture of, those of metals and nonmetals.

Double bond

consists of a sigma and a pi bond

Triple bond

consists of a sigma and two pi bonds.

Single bond

consists of one sigma bond

Azimuthal Quantum Number

describes the shape of orbital. It is denoted by l. Values of l are from zero to n-1. For s-orbital = 0 For p-orbita = 1 For d-orbital = 2 For f-orbital = 3 With the help of the value of azimuthal quantum number we can determine the total number of energy sub levels in a given energy level.

Spin Quantum Number

describes the spin of an electron in an orbital. It is denoted by 's'. It has only two possible values. s = +1/2 , S = -1/2 s = +1/2 for clockwise spin. s = -1/2 for anti-clockwise spin. An electron spinning clockwise is indicated by (arrow up) and anti-clockwise by (arrow down)

Magnetic Quantum Number

indicates the orientation of an orbital in space in an applied magnetic field . It is denoted by 'm'. Values of 'm' are from (-l) to (+l) through zero. Orbital s: l = 0, m = 0. Orbital p: l = 1, m = -1,0,+1. Orbital d: l = 2, m = -2,-1,0,+1,+2. Orbital f: l = 3, m = -3,-2,-1,0,+1,+2,+3.

Formal charge

is the charge assigned to an atom in a molecule, assuming that electrons in a chemical bond are shared equally between atoms, regardless of relative electronegativity. Formal charge = [# of valence electrons] - [electrons in lone pairs + 1/2 the number of bonding electrons].

Lone pairs

refers to a pair of valence electrons that are not shared with another atom[1] and is sometimes called a non-bonding pair.

Polarity

refers to a separation of electric charge leading to a molecule or its chemical groups having an electric dipole or multipole moment. Polar molecules interact through dipole-dipole intermolecular forces and hydrogen bonds. Molecular polarity is dependent on the difference in electronegativity between atoms in a compound.

Principal Quantum Number

represents the energy level of an electron. It is denoted by 'n' .The values of principle quantum number are from 1 to n (where n is a positive whole number i.e. n = 1,2,3,4,.........). It also describes the size of orbit. The maximum number of electrons in energy level n (electron shell n) is 2n^2.

Electron configuration

the distribution of electrons of an atom or molecule (or other physical structure) in atomic or molecular orbitals. In electron configuration notation, the first number denotes the principal energy level, the latter designates the subshell, and the superscript gives the nunmber of electrons in the subshells.

Covalent bonding

the stable balance of attractive and repulsive forces between atoms when they share electrons.