BONDING AND VSEPR THEORY

STRUCTURES OF SOLIDS AND LIQUIDS

Intermolecular Attractions

CHEMICAL BONDS – OCCUR WHEN ELECTRONS ARE TRANSFERRED OR SHARED BY ELEMENTS SO THAT THEY EACH BECOME MORE STABLE

Electron Dot (Lewis) Diagrams Explain Chemical

Bonding

How many electrons do most atoms want in their outer energy level to be stable?

0 1 2 6 8 18

17% 17% 17%17%17%17%

1. 02. 13. 24. 65. 86. 18

Bonds that form between two nonmetals are which type?

Ionic

Covalent

Metallic

33% 33%33%1. Ionic2. Covalent3. Metallic

Drawing Electron Dot Diagrams

Electrons usually stay in pairs when bonded.

Bonding pairs – pair of electrons that form the bond

- can be represented as a line segment

Lone (or unbonded) pairs – pairs of electrons that are not involved in bonds and are shown as dots

How many bonding pairs are in the following compound?

1 2 3 4 6 9 18

14% 14% 14% 14%14%14%14%1. 12. 23. 34. 45. 66. 97. 18

How many lone pairs are in the following compound?

1 2 3 4 6 9 18

14% 14% 14% 14%14%14%14%1. 12. 23. 34. 45. 66. 97. 18

How many bonding pairs and lone pairs are in the following compound?

6 bonding pair...

12 bonding pai...

12 bonding pai...

6bonding pairs

...

0% 0%0%0%

1. 6 bonding pairs, 18 lone pairs

2. 12 bonding pairs, 18 lone pairs

3. 12 bonding pairs, 36 lone pairs

4. 6bonding pairs, 6 lone pairs

Drawing Electron Dot Formulas for Compounds

Exceptions: Hydrogen only needs 2 electrons (1 bond)

Boron tends to need only 6 electrons (3 bonds)

Single atoms go in the center

If more than one single atom, middle atom central atom

Draw the electron dot formula. Then state how many bonding and unbonding pairs are present.

A) NBr3

B) Water

C) Chlorite ion (ClO2- )

D) CF2Cl2

Multiple Bonds

If there are not enough electrons to form full octets, multiple bonds may need to be formed.

Draw the electron dot formula

E) O2

F) CO2

Resonance Structures

If there are more than one possibility, resonance structures are drawn.

Resonance structures show possible locations of the bonds. In reality the electrons exist as an average of the two structures – splitting time equally between them.

Resonance Example

Each resonance structure is shown followed by the combination with the double bonds shown with a dotted line as one of the bonds.

Draw the electron dot formulas including resonance structures

G) SO2

H) N2O

What is the name of the property that describes the tendency of an atom to attract electrons when bonded to another atom?

Ioniza

tion ene...

Conductivit

y

Electr

onegativ..

.

Metallic

Chara...

Bond length

20% 20% 20%20%20%

1. Ionization energy

2. Conductivity3. Electronegativit

y4. Metallic

Character5. Bond length

Classifying Bond Types

Chemical bonds can be classified by how much the bonded electrons are shared or are not shared by the elements involved.

• Electronegativity: The ability of one atoms in a molecule to attract electrons to itself.

• Wolfgang Pauling set electronegativities on a scale from 0.7 (Cs) to 4.0 (F).

• Electronegativity increases • across a period and • down a group.

Electronegativities of Elements

Electronegativity

Bond Classification based on Electronegativity Difference

As the difference in electronegativity increases, electrons are less equally shared and become more polar.

Type of Bonds Electronegativity Difference

Nonpolar covalent

Polar covalent

Ionic

x ≤ 0.4

0.4 < x < 1.8

x ≥ 1.8

Bond Classification based on Electronegativity Difference

Classify the bond between the following elements: Cl and Cs

Ionic

Polar Covale

nt

Nonpolar Cova

l...

0% 0%0%

1. Ionic2. Polar Covalent3. Nonpolar

Covalent

Classify the bond between the following elements: C and H

Ionic

Polar Covale

nt

Nonpolar Cova

l...

0% 0%0%

1. Ionic2. Polar Covalent3. Nonpolar

Covalent

Classify the bond between the following elements: N and O

Ionic

Polar Covale

nt

Nonpolar Cova

l...

0% 0%0%

1. Ionic2. Polar Covalent3. Nonpolar

Covalent

INTERMOLECULAR FORCES

Interactions between Molecules

Polarity of a Compound

Like bonds, compounds themselves can also be classified as polar or nonpolar.

Polarity is based on: Difference in electronegativity of atoms within a

compound Symmetry of the compound

Nonpolar Compounds

- Diatomic molecules are always nonpolar. (ex. F2)

- Also, compounds that are totally symmetric may be nonpolar as well. (ex. CCl4)

Nonpolar Compound – the bonds are polar but the dipoles cancel out since the compound is symmetrical (tetrahedral)

Nonpolar Compound – the bonds are polar but the dipoles cancel out since the compound is symmetrical (linear)

Polar Compounds

Polar compounds have one side of the compound that is more positive and another side that is more negative.

Polar Compounds

Polar Compounds

Polar Compounds

BF3 = Polar or Nonpolar

1 2

0%0%

1. Polar2. Nonpolar

CH3F = Polar or Nonpolar?

1 2

0%0%

1. Polar2. Nonpolar

CF4 = Polar or Nonpolar?

1 2

0%0%

1. Polar2. Nonpolar

Br2 = Polar or Nonpolar

1 2

0%0%

1. Polar2. Nonpolar

PBr3 = Polar or Nonpolar

1 2

0%0%

1. Polar2. Nonpolar

Intermolecular Forces

Intermolecular Forces are forces that exist between two molecules that hold them together.

Intermolecular Forces are caused by charge differences and polarity (because positive and negatives attract)

The stronger the polarity, the stronger the attraction between molecules.

Intermolecular Forces

The stronger the polarity, the stronger the attraction between molecules.

The strength of the attraction between molecules determines properties such as: Boiling point Melting point Surface tension Cohesion Capillary action

Types of Intermolecular Forces

Three major types of intermolecular forces: Dipole-Dipole Interactions

Hydrogen Bonds

Dispersion Forces

Dipole-Dipole Interaction

Occurs in polar molecules.

Positive pole of one molecule is attracted to the negative pole of the next molecule.

Hydrogen Bonds

Occurs in polar molecules when the hydrogen atom is attracted to the more electronegative nitrogen, oxygen, or fluorine atom of another molecule.

Dispersion Forces

Dispersion forces are the weakest type of intermolecular forces because they exist between nonpolar molecules.

Usually, the electrons are shared equally. But because electrons are constantly moving, sometimes a temporary dipole forms when all the electrons are on one side of the molecule.

This temporary dipole would cause an attraction with another temporary dipole.

Summary of Intermolecular Forces (from strongest to weakest)

What kind of intermolecular force would exist in H2O?

1 2 3

0% 0%0%

1. Hydrogen bonding

2. Dipole-Dipole3. Dispersion

What kind of intermolecular force would exist in PCl3?

1 2 3

0% 0%0%

1. Hydrogen bonding

2. Dipole-Dipole3. Dispersion

What kind of intermolecular force would exist in Br2?

1 2 3

0% 0%0%

1. Hydrogen bonding

2. Dipole-Dipole3. Dispersion

What kind of intermolecular force would exist in NH3?

1 2 3

0% 0%0%

1. Hydrogen bonding

2. Dipole-Dipole3. Dispersion

Melting and Boiling Points

In order to melt or boil a substance, intermolecular forces must be broken.

Therefore, weaker intermolecular forces require less energy and have lower melting and boiling points.

Therefore, stronger intermolecular forces require more energy and have higher melting and boiling points.

Which type of intermolecular force would have the lowest boiling point and exist as a gas at room temperature?

1 2 3 4 5

0% 0% 0%0%0%

1. Ionic Bonds2. Dispersion

Forces3. Hydrogen

Bonds4. Covalent Bonds5. Dipole-Dipole

attractions

Which type of intermolecular force would have the highest melting point and always exist as a solid at room temperature?

1 2 3 4 5

0% 0% 0%0%0%

1. Ionic Bonds2. Dispersion

Forces3. Hydrogen

Bonds4. Covalent Bonds5. Dipole-Dipole

attractions

Which compound would have the lowest boiling point and exist as a gas at room temperature?

1 2 3 4 5

0% 0% 0%0%0%

1. H2O2. N2

3. CuI2

4. CO2

5. LiF

Which compound would have the highest melting point?

1 2 3 4 5

0% 0% 0%0%0%

1. NH3

2. NaBr3. F2

4. CO2

5. NH3

Effects of Molecular Mass on Melting and Boiling Points

The higher molecular masses will have higher melting and boiling points because they have more electrons that form temporary dipoles.

Acts as the tiebreaker if the forces are the same types – highest mass has the strongest force (highest melting and boiling point).