Chapter 6 - Shapes of Molecules and Intermolecular Forces

Question 1

The shapes of molecules an ions are determined by applying valence shell electron pair repulsion theory.
Describe this theory.

Answer 1

Valence shell electrons are the electrons in the outermost shell.
The electron pairs in the valence shell repel each other as far apart as possible.
Multiple bonds have a similar repelling effect to single bonds.
Lone pairs of electrons repel more than bonded pairs of electrons reducing bond angles by 2.5°.

Question 2

What is the shape and bond angle of a molecule with 2 bonded pairs/regions?

Answer 2

Linear

180°

Question 3

What is the shape and bond angle of a molecule with 3 bonded pairs/regions of electrons?

Answer 3

Trigonal Planar

120°

Question 4

What is the shape and bond angle of a molecule with 4 bonded pairs/regions of electrons?

Answer 4

Tetrahedral

109.5°

Question 5

What is the shape and bond angle of a molecule with 6 bonded pairs/regions of electrons?

Answer 5

Octahedral

90°

Question 6

What is the shape and bond angle of a molecule with 3 bonded pairs/regions of electrons and 1 lone pair of electrons?

Answer 6

Pyramidal

107°

Question 7

What is the shape and bond angle of a molecule with 2 bonded pairs/regions of electrons and 2 lone pairs of electrons?

Answer 7

Non-linear

104.5°

Question 8

By how much does a lone pair reduce the bond angle?

Answer 8

2.5°

Question 9

What is electronegativity?

Answer 9

The ability of an atom to attract the bonding electrons in a covalent bond.

Question 10

The closer an element is to _______, the more electronegative the element.

Answer 10

The closer an element is to fluorine, the more electronegative the element.

Question 11

What is a polar bond?

Answer 11

A covalent bond in which the shared pair of electrons is shared unequally between the bonding atoms due to their differences in electronegativity.

Question 12

What is a dipole?

Answer 12

The separation of partial charges in a molecule.

Question 13

If a molecule is symmetrical, is it polar or non-polar?

Answer 13

Non-polar as the dipoles in the polar bonds cancel each other out.

Question 14

What are the three types of intermolecular force?

Answer 14

• Induced dipole-dipole interactions (London forces)
• Permanent dipole-dipole interactions
• Hydrogen bonds (special type of permanent dipole-dipole interactions)

Question 15

Describe and explain induced dipole-dipole interactions (London forces)?

Answer 15

Electrons in a molecule are constantly moving.
At any instant in time, the distribution of electrons may be uneven. As a result, a molecule may have a temporary dipole.
The presence of a temporary dipole in one molecule can cause an induced dipole to form in a nearby molecule. The induced dipole can then induce a dipole in another neighbouring molecule.

Question 16

How does the number of electrons affect melting and boiling points.

Answer 16

The more electrons a molecule has, the stronger the London forces between molecules. More energy will be needed to break the intermolecular forces, increasing the melting and boiling points.

Question 17

What is a permanent dipole?

Answer 17

The dipole in a polar molecule.

Question 18

Describe permanent dipole-dipole interactions.

Answer 18

Polar molecules contain regions with different electron densities.
When two polar molecules are close, they will attract one another. This attraction exists between any two molecules that have permanent dipoles, in addition to London forces.

Question 19

The more _______ a molecule has, the stronger the London forces.

Answer 19

The more electrons a molecule has, the stronger the London forces.

Question 20

Describe the structure of simple molecular lattices.

Answer 20

In the solid state, simple molecules exist as a simple molecular lattice which contains molecules held together in a regular structure:

• The atoms within each molecule are bonded together by strong covalent bonds.
• The molecules are held in place by weak intermolecular forces.

Question 21

What are the properties of simple molecular substances?

Answer 21

• Low melting and boiling point
• Soluble in non-polar solvents
• Do not conduct electricity (as they have no charge)

Question 22

What are the properties of giant covalent substances?

Answer 22

• High melting and boiling point
• Insoluble in non-polar and polar solvents
• Non-conductors of electricity (only exception is graphite)

Question 23

What are the shapes of symmetrical (non-polar molecules)?

Answer 23

Linear
Trigonal Planar
Tetrahedral

Question 24

Why do simple molecular lattices not exist in liquid or gas states?

Answer 24

Because in the liquid and gas state, intermolecular forces are constantly breaking and reforming.
In solid molecules, these attractions hold the molecules in a fixed position (simple molecular lattice).

Question 25

State and explain the unique properties of water.

Answer 25

Ice is less dense than water. This is because the molecules in ice are held in an open lattice structure. When ice melts, the open lattice structure collapses.

Relatively high melting and boiling point as hydrogen bonds are stronger than other intermolecular forces.

Question 26

Hydrogen bonds occur between:

Answer 26

• A H atom bonded to an electronegative atom (O, N, or F) within a molecule.
• A H atom and a lone pair of electrons on a highly electronegative atom from a different molecule.

Question 27

What are hydrogen bonds?

Answer 27

Especially strong permanent dipole-dipole interactions.