Chapter 09: Chemical Bonding I: The Lewis Model

Question 1

Covalent bond

Answer 1

Electrons shared

Nonmetal + nonmetal

Usually polar, sometimes nonpolar

Question 2

Metallic bonding

Answer 2

Electrons pooled (“sea of electrons”)

Metal + metal

Results from attraction of cations to delocalized electrons

Question 3

Ionic bond

Answer 3

Electrons transferred

Oppositely charged ions bond, lowering overall potential energy

Question 4

Lewis structure of ionic bond

Answer 4

Cation + anion in squared brackets

Question 5

Exceptions to octet rule

Answer 5

1. Duet rule (H, Li, Be, B – first four elements)
2. Expanded octet (period 3 and below)

Question 6

Lattice energy

Answer 6

Exothermic energy associated with formation of crystalline lattice

Electrostatic attraction is nondirectional (no direct anion-cation pair)

Hence, no ionic molecule

Depends directly on size;
inversely on distance between ions

Question 7

Born-Haber cycle

Answer 7

Hypothetical series of steps representing the formation of an ionic compound from constituent elements

Change in enthalpy known for each step except last (for lattice energy)

Utilize Hess’ law to determine lattice energy enthalpy change

Question 8

Ion size & lattice energy

Answer 8

Force of attraction inversely proportional to distance between them

Larger ionic radius = weaker attraction = smaller lattice energy

Less exothermic with increasing ionic radius

Question 9

Ion charge & lattice energy

Answer 9

More exothermic with increasing ionic charge

Force of attraction directly proportional to product of charges

Larger charge = stronger attraction = larger lattice energy

*Generally more important than ion size

Question 10

Bonding pair

Answer 10

Electrons that are shared by atoms

Question 11

Lone pair

Answer 11

Nonbonding pair

Electrons not shared by atoms

Question 12

Polar covalent bond

Answer 12

Covlanet bond between unlike atoms

Unequal sharing of electrons

One atom pulls electrons in bond closer to its side

Question 13

Electronegativity

Answer 13

EN

Ability of an atom to attract electrons to itself in a chemical bond

Measure of EN is relative

EN increases toward the right of a period and up a group

F = most EN; Fr = least EN

Noble gases = NO EN

EN inversely related to atomic size

Question 14

Pure/nonpolar covalent bond

Answer 14

Electrons shared equally (between atoms with like EN)

Question 15

Dipole moment

Answer 15

A measure of bond polarity

i.e. the polarity of a bond between two atoms within a molecule or ion

Directly proportional to size of particle charges and distance between charges

Represented with vector arrow

The more electrons shared & the larger the atoms are, the larger the dipole moment

Question 16

Percent ionic character

Answer 16

Percentage of a bond’s measured dipole moment compared to what it would be if the electron were completely treansferred

Indicates the degree to which the electron is transferred

Question 17

Writing Lewis structures (4)

Answer 17

1. Write skeletal structure (H is always terminal; more electronegative elements are terminal)
2. Calculate total # of valence electrons
3. Distribute electrons
4. If any atoms lack an octet, form double or triple bonds as necessary

Question 18

Formal charge (4)

Answer 18

Charge an atom would have if all bonding electrons were shared equally

FC of an atom = Valence e^- - nonbonding e^- - (1/2)(bonding e^-)

1. Sum of all formal charges in neutral molecule = 0
2. Sum of all formal charges in ion = ion charge
3. Small (or zero) formal charges on individual atoms are better than large ones
4. When formal charge cannot be avoided, negative formal charge should reside on the most electronegative atom

Question 19

Resonance structures

Answer 19

Based on delocalization of electrons which stablilizes molecule/ion

Multiple correct Lewis structures that average out to what occurs in nature

Same atomic structure, different bond arrangement

*Formal charges must total the same in each resonance structure

Question 20

Resonance hybrid

Answer 20

A combination of resonance forms that best represents that actual molecule (cannot be drawn as one structure)

Question 21

Free radicals

Answer 21

AKA odd-electron species

Molecules and ions with an odd number of electrons in Lewis structures

Reactive because of odd number of electrons

Question 22

Incomplete octet

Answer 22

Applies to first five elements: H, He, Li, Be, B

Question 23

Expanded octets

Answer 23

Applies to elements in third row and below

Up to 12 (sometimes 14) electrons

Stored in d orbitals

Question 24

Bond energy

Answer 24

The amount of energy required to break one mole of a bond in the gas phase

*Average bond energies can be used to estimate ΔH_rxn

ΔH_rxn = Σ(ΔH bonds broken) + Σ(ΔH bonds formed)

broken = positive

formed = negative

Question 25

Periodic trends in bond energies (4)

Answer 25

1. The more electrons shared between two like atoms, the stronger the bond
2. The shorter the bond, the stronger the bond
3. Bonds get stronger up a group
4. Bonds get stronger toward the right of a period

Question 26

Bond length

Answer 26

Distance between nuclei of bonded atoms

Question 27

Average bond length

Answer 27

Average for similar bonds from many compounds

Used because the actual bond length depends on the other atoms around the bond

Question 28

Periodic trends in bond lengths (4)

Answer 28

1. The more electrons shared, the shorter the bond
2. The longer the bond, the weaker
3. Bond length increases to the left of a period
4. Bond length increases down a group

Question 29

Electron sea model

Answer 29

Simplest theory of metallic bonding

Metal atoms release valence electrons to be shared as a pool