Summary Chapter 3-4

General, Organic, & Biological ChemistryJanice Gorzynski Smith

CHAPTER 3-4: Concepts to Know

The difference between ionic and covalent bonds Define cations and anions Predict cation/anion charge using the octet rule or group number

Familiar with metals with multiple potential charges (do not need to memorize)

Determine ionic compound formulas from the name of a compound or from the elements that compose it. Criss-cross rule

Naming ionic compounds and covalent molecules Familiar with polyatomic ions (do not need to memorize but must

be able to recognize) Draw lewis dot structures Determine molecular geometry Identify polar bonds Determine dipole moment of molecules

Need to Memorize

Ionic vs Covalent Bonding

Ionic Bonds result from electrostatic attraction between a cation and anion: metal-nonmetal (with the

exception of NH4+ and H3O+ cations).

Covalent bonds result from the sharing of electronsbetween two atoms: nonmetal-nonmetal.

Li FIonic Bonds Covalent Bonds

Naming

HOW TO Name an Ionic Compound

Step [1]

Determine the charge on the cation.

Step [2]

Name the cation and the anion If the cation could be

multiple charges indicate the charge with roman numerals or with a –ous / -ic suffix.

Step [3]

Write the name of the cation first then the name of the anion

HOW TO Name a Covalent Molecule

Step [1]

Name the first nonmetal by its element name and the second using the suffix“-ide.”

Step [2]

Add prefixes to show the number of atoms of each element.

Predicting Cations & Anions

the anion charge = 8 – group number

the cation charge = the group number

Octet Rule

only 6 e− on B

B

F

FF

10 e− on P 12 e− on S

S

O

OHHO

O

P

O

OHHO

OH

The octet rule: a main group element is especially stable when it possesses an octet of e− in its outer shell.

octet = 8 valence e−

Exceptions (need to memorize):

Ionic Compound FormulasHOW TO Write a Formula for an Ionic

Compound

Step [1]

Identify which element is the cationand which is the anion.

Step [2]

Determine how many of each ion type is needed for an overall charge of zero. When the cation and anion have different

charges, use the ion charges to determine the number of ions of each needed.

Step [3]

To write the formula, place the cationfirst and then the anion, and omit charges.

“Criss-cross” rule Make magnitude of charge on one

ion into subscript for other When doing this, make sure that

subscripts are reduced to lowest whole number.

Al2O3Al3+ O2–

Lewis Dot Structures

Step [1]

Step [2]

Step [3]

Arrange the atoms next to each other that you think are bonded together. Place H and halogens on the periphery, since they can only form one bond.

Count the valence electrons. The sum gives the total number of e− that must be used in the Lewis structure. For each atom the number of bonds = 8 – valence electrons.

Arrange the electrons around the atoms. Place one bond (two e−) between every two atoms. Use all remaining electrons to fill octets with lone pairs, beginning with atoms on the periphery.

NH3

N HH H

Nitrogen has 5 valence electrons, so it will have 8 – 5 = 3 bonds.

Hydrogen will have 2-1 = 1 bond.There are 8 total valance electrons

H NH

H1 lone pair:

2

3 bonds:

6

Total e-

8

= total valence e-

Resonance StructuresResonance structures exist when there are multiple lewis dot structures with different electron arrangements with the same connectivity between atoms. Resonance structures help us understand delocalization (spreading) of charge within a molecule that stabilizes the anion or cation.

Negative charge not localized on 2 of 3 oxygen, but instead delocalized and spread over entire molecule.

NO

YES

Other Examples: CO32- and O3

Molecular Shape

Periodic Trend: Electronegativity

ElectronegativityINCREASING

Polarity

1. Assess the relative electronegativity of atoms bonded together, if there is a difference it is a polar bond.

2. Indicate polar bonds with δ+ / δ - or3. If polarity of bonds does not cancel draw the overall

dipole moment of the molecule using

Electron density is disproportionately distributed over the molecule. Above red indicates partial negative charge, or greater electron density, and blue indicates partial positive charge.Effectively oxygen is hogging the electrons

Practice Problems: Lewis Dot StructuresFor each molecule draw a lewis dot structure, some have several correct answers,

try to figure them all out. Watch out some have multiple bonds. And the last molecule is an anion, find all resonance structures.

CH4S C5H10 C3H8O C2H3N

C2H3O2-

Solutions will be posted separately, in the solutions folder on my facweb site.