Ch. 3 - Structure and Properties of Ionic and Covalent Compounds

Nonpolar molecule

- Any molecule made up of only nonpolar bonds is a nonpolar molecule

Features of Covalent Bonds (3):

- Covalent bonds form between atoms with similar tendencies to gain or lose electrons - Compounds containing covalent bonds are called covalent compounds or molecules - The diatomic elements have completely covalent bonds (totally equal sharing), as a truly covalent bond can only occur when both atoms are identical

Covalent compounds

- Covalent compounds are typically formed from nonmetals - Not a part of a massive 3-dimensional crystal structure - Exist as discrete molecules in the solid, liquid, and gas states

Writing Formulas of Ionic Compounds from the Names of the Compounds

- Determine the charge of each ion - Write the formula so that the resulting compound is neutral

Writing Formulas of Ionic Compounds from the Identities of the Component Ions

- Determine the charge of each ion -- Metals have a charge equal to group number -- Nonmetals have a charge equal to the group number minus 8 - Cations and anions must combine to give a formula with a net charge of zero if neutral -- If neutral, it must have the same number of positive charges as negative charges

Ionic Compounds

- Metals and nonmetals usually react to form ionic compounds - The metals are cations and the nonmetals are anions - The cations and anions arrange themselves in a regular 3-dimensional repeating array called a crystal lattice - Formula of an ionic compound is the smallest whole-number ratio of ions in the substance

The essential features of ionic bonding are the following (5):

- Metals tend to form cations because they have low ionization energies and low electron affinities - Nonmetals tend to form anions because they have high ionization energies and high electron affinities - Ions are formed by the transfer of electrons - The oppositely charged ions formed are held together by an electrostatic force - Reactions between metals and nonmetals tend to form ionic compounds

To determine if a molecule with polar bonds is polar or nonpolar, draw the Lewis structure and follow these guidelines (4):

- Molecules that have no lone pair on the central atom and where all terminal atoms are the same are nonpolar - Molecules with one lone pair on the central atom are polar - Molecules that have more than one lone pair on the central atom are usually polar, but there are exceptions - Molecules that are made up of only carbon and hydrogen (hydrocarbons) are nonpolar

Some molecules have exceptions to the octet rule; these include:

- Odd-electron compounds -- It is impossible to pair all electrons to achieve an octet simply because the compound contains an odd number of valence electrons - Incomplete octets - Expanded octets -- Elements in the third period and beyond may involve d orbitals and form an expanded octet with 10 or 12 electrons surrounding the central atom

Writing Lewis Symbols

- Place one dot on each side until there are 4 dots around the symbol - Then add a second dot to each side in turn - The number of valence electrons limits the number of dots placed - Each unpaired dot (unpaired valence electron) is available to form a chemical bond

Factors influencing boiling and melting points (3)

- Strength of the attractive force holding the substance in its current physical state - Molecular mass: Larger molecules have higher m.p. and b.p. than smaller molecules as it is more difficult to convert a larger mass to another phase - Polarity: Polar molecules have higher m.p. and b.p. than nonpolar molecules of similar molecular mass due to their stronger attractive force

The most electronegative elements are found: The least electronegative elements are found:

- The most electronegative elements are found in the upper right corner of the periodic table - The least electronegative elements are found in the lower left corner of the periodic table

Naming Acids

- The names of acids are related to the names of anions: -- Binary acids: -- () -ide becomes hydro-...-ic acid -- Oxyacids (acids from polyatomic ions): -- () -ate becomes -ic acid -- () -ite becomes -ous acid -- () The name of the acid also contains the per- or hypo- prefixes contained in the name of the ion

5 conventions for naming covalent compounds:

- The names of the elements are written in the order in which they appear in the formula - A prefix indicating the number of each kind of atom found in the unit is placed before the name of the element - If only one atom of a particular kind is the first element present in the molecule, the prefix mono- is usually omitted from that first element - The stem of the name of the last element is used with the suffix -ide - The final vowel in a prefix is often dropped before oxide

Boiling point

- The temperature at which a liquid is converted to a gas at a specified pressure - ionic compounds have higher boiling points than covalent compounds

Basic procedure to determine molecular shape

- Write the Lewis structure - Count the number of bonded atoms and lone pairs around the central atom - If no lone pairs are present, geometry is: -- 2 bonded atoms: linear -- 3 bonded atoms: trigonal planar -- 4 bonded atoms: tetrahedral - If there are lone pairs, look at the arrangement of the atoms and name the geometry; either bent or trigonal pyramidal

Single bond

- a bond in which 1 pair of electrons is shared by 2 atoms

Double bond

- a bond in which 2 pairs of electrons are shared by 2 atoms

Triple bond

- a bond in which 3 pairs of electrons are shared by 2 atoms

Resonance

- a condition that occurs when more than one valid Lewis structure can be written for a particular molecule - Two or more Lewis structures contribute to the real structure - None of the possible resonance Lewis structures exist, the true structure is a resonance hybrid of the drawn resonance forms - Stability of a compound increases with the number of resonance structures that can be drawn

Electronegativity

- a measure of the ability/tendency of an atom in a molecule to attract shared electrons in a chemical bond - Elements with high electronegativity have a greater ability to attract electrons than do elements with low electronegativity - the difference in electronegativity is used in classifying chemical bonds and the extent of bond polarity -- the greater the difference in electronegativity between two atoms, the greater the polarity of their bond

Valence-shell electron-pair repulsion (VSEPR) theory

- a model that predicts molecular geometry using the premise that electron pairs will arrange themselves as far apart as possible, to minimize electron repulsion -- In the covalent bond, bonding electrons are localized around the nucleus -- The covalent bond is directional, having a specific orientation in space between the bonded atoms -- Ionic bonds have electrostatic forces which have no specific orientation in space

Tetrahedral

- a molecular geometry consisting of 4 groups attached to a central atom that occupy the 4 corners of an imagined regular tetrahedron - 109.5 degree bond angles - This is the primary structure of a full octet

Trigonal planar

- a molecular geometry in which a central atom is bonded to 3 atoms that lie at the vertices of an equilateral triangle - All atoms lie within one plane and all bond angles are 120 degrees - Used if there are no lone pair electrons on the central atom

Trigonal pyramidal

- a nonplanar structure involving 3 groups bonded to a central atom with one lone pair in which each group is equidistant from the central atom - The lone pair takes one of the corners of the tetrahedron without being visible, distorting the arrangement of electron pairs - ~107 degree bond angles - Used if there is one lone pair at the central atom, because a lone pair is more electronegative with a greater electron repulsion

Bent

- a planar molecule with bond angles other than 180 degrees - two bonded groups with some number of lone pairs on the central atom

Nonelectrolytes

- a substance that, when dissolved in water, produces a solution that does not conduct an electric current - As with dissolved covalently bonded molecules as they do not dissociate into ions

Nomenclature

- a system involving the assignment of a correct and unambiguous name to each and every chemical compound

Crystal lattice

- a unit of a solid characterized by a regular, 3-dimensional, repeating arrangement of components, as in ionic bonding

Lone pair

- an electron pair shown around an atom in a Lewis structure that is not shared/involved in bonding

Ionic bond

- an electrostatic attractive force (due to the opposite charges of the ions) between cations and anions resulting from the transfer of one or more electrons from one atom to another - Representative elements form ions that obey the octet rule - Electrons are lost by a metal and gained by a nonmetal - Each atom achieves a "noble gas" configuration - 2 ions are formed: a cation and an anion, which are attracted to each other - These ions arrange in a crystal lattice

monatomic ion

- an ion formed by electron gain or loss from a single atom - charge can be determined from the periodic table for main-group elements

Electrolytes

- an ionic material that dissolves in water to produce an electrolytic solution that conducts an electric current

Intermolecular force

- any attractive force that occurs between molecules - Determine many physical properties such as the solubility of one substance in another and the freezing and boiling points of liquids, but they are a direct consequence of the intramolecular forces within the individual units/molecules

Intramolecular force

- any attractive force that occurs within a molecule - Chemical bonds that determine the shape and polarity of individual molecules

Covalent bonds

- attractive force due to the sharing of electrons between atoms - form molecules

Polar covalent bond

- bonds made up of unequally shared electron pairs - There is a somewhat positive end (partial positive), denoted with a δ+ symbol - There is a somewhat negative end (partial negative), denoted with a δ- symbol

diatomic molecule

- composed of two identical atoms joined by a covalent bond

Molecules

- compounds characterized by covalent bonding - Not a part of a massive 3-dimensional crystal structure - Exist as discrete molecules in the solid, liquid, and gas states

Isomers

- compounds that have the same molecular formula but have different chemical structures - Hydrocarbons have many possible isomers

Covalent compounds properties (4)

- covalent compounds may be solids, liquids, or gases at room temperature - covalent compounds have lower melting and boiling points than ionic compounds because their bonds are easier to break - covalent compounds may either be crystalline, have a regular structure, or amorphous, have no regular structure - When covalent compounds dissolve in water, the compound does not dissociate and the molecules retain their neutral characteristics (do not conduct electricity - nonelectrolytes)

Expanded octets

- exception to the octet rule - Elements in the third period and beyond may involve d orbitals and form an expanded octet with 10 or 12 electrons surrounding the central atom

Odd-electron compounds

- exception to the octet rule - It is impossible to pair all electrons to achieve an octet simply because the compound contains an odd number of valence electrons

Dipole

- having two "poles" or ends, one pole is more negative and the other pole is more positive (polar molecules) - Polar molecules placed in an electric field align themselves with the field - Nonpolar molecules don't align themselves in an electric field

Polyatomic ions

- ions composed of 2 or more atoms bonded together with an overall positive or negative charge - Within the ion itself, the atoms are bonded using covalent bonds - The positive and negative polyatomic ions will be bonded to each other with ionic bonds

Possible geometries of molecules (5)

- linear - trigonal planar - tetrahedral - bent - trigonal pyramidal

Covalent bonding is subdivided into 2 classes:

- polar covalent - nonpolar covalent

dissociation

- production of positive and negative ions when a soluble ionic compound dissolves in water, forming electrolytes

Lewis structures

- representation of a molecule (or polyatomic ion) that shows valence electron arrangement among the atoms in a molecule (or polyatomic ion) to satisfy the octet rule

Lewis symbol

- representation of an atom (or ion) using the atomic symbol (for the nucleus and core electrons) and dots to represent valence electrons - As only valence electrons participate in bonding, this makes it much easier to work with the octet rule - The number of dots used corresponds directly to the number of valence electrons located in the outermost shell of the atoms of the element

Bond energy

- the amount of energy necessary to break a chemical bond holding 2 atoms together - Measure of the stability of a bond - The bond energy trend is: single<double<triple

Bond length

- the distance separating the nuclei of two adjacent atoms - The bond length trend is: triple<double<single

chemical bond

- the force of attraction holding two atomic nuclei together in a chemical compound - This attractive force overcomes the repulsion of the positively charged nuclei of the two atoms participating in the bond - Interactions involving valence electrons are responsible for the chemical bond

Solubility

- the maximum amount of solute that dissolves in a given amount of solvent at a specific temperature - Polar molecules are most soluble in polar solvents - Nonpolar molecules are most soluble in nonpolar solvents

Formula

- the representation of the fundamental compound unit using chemical symbols and numerical subscripts - The formula identifies the number and type of the various atoms that make up the compound unit - The number of like atoms in the unit is shown by the use of a subscript - Presence of only one atom is understood when no subscript is present

Linear

- the structure of a molecule in which the bond angle(s) about the central atom(s) is (are) 180 degrees

Melting point

- the temperature at which a solid converts to a liquid - ionic compounds have higher melting points than covalent compounds

Ionic compound properties (4)

- usually solids at room temperature - have higher melting and boiling points than covalent compounds because considerable energy is required to break apart the uncountable numbers of ionic interactions (electrostatic attractions) within an ionic crystal lattice - Ionic solids/compounds are crystalline, characterized by a regular structure - When ionic compounds dissolve in water, the ions dissociate (form positive and negative ions in solution) and the solution conducts electricity

Naming oxyacids (acids from polyatomic ions) - with -ate ending - with -ite ending

-ate becomes -ic acid -ite becomes -ous acid The name of the acid also contains the per- or hypo- prefixes contained in the name of the ion

Naming binary acids with -ide ending

-ide becomes hydro-...-ic acid

Drawing Lewis Structures of Covalent Compounds

1. Draw a skeletal structure of the molecule 2. Arrange the atoms in their most probable order by electronegativity (least electronegative is the central atom) 3. Find the number of valence electrons for each atom and the total for the compound 4. Use electron pairs to connect each element with a single bond 5. Place electron pairs around the atoms 6. Redistribute the electrons (and potentially use double/triple bonds) to satisfy the octet rule and the total number of electrons

Lewis Structure Guidelines

1. Use chemical symbols for the various elements to write the skeletal structure of the compound - The least electronegative atom will usually be the central atom and often the element written first in the formula -- Element farthest to the left and/or lowest in the periodic table - Hydrogen and fluorine must occupy terminal (non-central) positions - Halogens occupy terminal positions, except when more electronegative elements are present - Carbon often forms chains of carbon-carbon covalent bonds 2. Determine the number of valence electrons associated with each atom in the compound; combine these valence electrons to determine the total number of valence electrons in the compound - For polyatomic cations, subtract one electron for each unit of positive charge to account for the fact that the positive charge arises from electron loss - For polyatomic anions, add one electron for each unit of negative charge to account for excess negative charge resulting from electron gain 3. Connect the central atom to each of the surrounding atoms with single bonds - Connection of the atoms uses 2 electrons per bond, keep track of how many you use 4. Place electrons not involved in bonding as lone pairs around the terminal atoms to complete the octet for each - If you have electrons left over after satisfying the octet rule for the terminal atoms, do the same for the central atom 5. If the octet rule is not satisfied for all the atoms, move one or more lone pairs on a terminal atom in to make a bond with the central atom, making double/triple bonds to ensure that all atoms have an octet 6. After you are satisfied with the Lewis structure that you have constructed, perform a final electron count to make sure they match the ones that were available

Common nomenclature system for ions with multiple charged forms

In the common nomenclature system, the suffix -ous indicates the lower ionic charge, and the suffix -ic indicates the higher ionic charge

The Stock system for ions with multiple charged forms

The stock system uses a Roman numeral placed immediately after the name of the ion to denote the charge on it, when required; this system is preferred

Writing formulas of covalent compounds

When writing formulas for covalent compounds, use the prefix before the element name to determine the subscript for the element

crystalline

characterized by a regular structure

Polar covalent electronegativity difference

electronegativity difference between 0.5 and 2.0

Nonpolar covalent electronegativity difference

electronegativity difference of 0.5 or less

Ionic electronegativity difference

electronegativity difference of 2.0 or larger

amorphus

have no regular structure

Two principal classes of chemical bonds exist:

ionic and covalent - Both involve valence electrons - Some bonds have characteristics of both types and are not easily identified as one or the other

When two or more Lewis structures contribute to the molecular structure, the compound displays

resonance