✨Module 2: Shapes of molecules and intermolecular forces

Question 1

The shape of a molecule is determined by …

Answer 1

The number of electron pairs in the central atom.

Question 2

What is the shape of ammonia and why?

Answer 2

Pyramidal. There are 3 bonded pairs and 1 lone pair. Electron pairs repel. Lone pairs repel more than bonded pairs. The bond angle is 107.

Question 3

What is the electron pair repulsion theory?

Answer 3

Electron pairs in central atom repel each other as far apart as possible. This holds the bonded atoms in a definite shape.

Question 4

If it’s not just one pair of electrons …

Answer 4

Then each multiple bond is treated as a bonding region.

Question 5

Lone pair - lone pair
Bonded pair - lone pair
Bonded pair - bonded pair
Which pair repels the most and why?

Answer 5

Lone pair - lone pair. Lone pair of electrons are slightly closer o central atom and occupies more space.

Question 6

For each lone pair …

Answer 6

The bond angle decreases by 2.5 degrees

Question 7

What is the shape of carbon dioxide and why?

Answer 7

Linear. 2 bonding regions repel one another as far apart as possible. Bond angle 180 degrees.

Question 8

Define electronegativity.

Answer 8

The ability of an atom to attract bonding electrons in a covalent bond towards itself. So the greater electronegativity means the more it attracts the electron pair towards itself.

Question 9

List factors affecting electronegativity. Explain each.

Answer 9

Nuclear charge/no. protons in nucleus - an increase in number of protons leads to an increase in nuclear attraction for the electrons in the outer shells.

Atomic radius (distance from nucleus to outermost shell) - electrons further away from the nucleus are less strongly attracted towards the nucleus, so an increased atomic radius results in decreased electronegativity.

Electron shielding -

Question 10

What is the Pauling scale?

Answer 10

To measure electronegativity on an atom.

Question 11

Why does electronegativity decrease down a group?

Answer 11

No. shells increases so more electron shielding, atomic radius increases. So there is decrease in attraction between the nucleus and outer bonding electrons. (increase in nuclear charge is negligible).

Question 12

Why does electronegativity increase across a period?

Answer 12

Nuclear charge increases, no new shells are added so atomic radius decreases. greater attraction between nucleus and bonded electrons.

Question 13

No difference between electronegativity.

Answer 13

Non polar covalent bond (can be pure if atoms bonded are same element).

Question 14

Small difference in electronegativity.
0 - 1.8

Answer 14

Polar covalent bond.

Question 15

Large difference in electronegativity.
Greater than 1.8

Answer 15

Ionic bond.

Question 16

Three types of intermolecular forces (weak interactions between dipoles of different molecules).

Answer 16

1. Induced dipole-dipole (London forces)
2. Permanent dipole-dipole forces
3. Hydrogen bonding.

Gets stronger as you go from 1 to 3.

Question 17

What are London forces/induced dipole-dipole interactions?

Answer 17

Weak intermolecular forces that exist between ALL molecules, polar or non-polar. They act between induced dipoles.

Question 18

How is an induced dipole-dipole force created?

Answer 18

Movement of electrons produces a changing dipole in a molecule.
Instantaneous dipole exists for a split second and this induces a dipole on the neighbouring molecule.
The induced dipole induces further dipoles on neighbouring molecules, which attract one another.

Question 19

The larger the number of electrons …

Answer 19

Larger induced dipoles, so more energy is needed to overcome the intermolecular forces, increasing bp.

Question 20

Explain why hydrogen chloride has a higher bp than F2.

Answer 20

Fluorine molecules are non-polar and only have London forces between molecules. HCL molecules are polar and have London forces and permanent dipole forces between molecules. Extra energy is needed to break additional permanent dipole forces between HCL molecules.

Question 21

Properties of simple molecular substances.

Answer 21

Low bmp as they have weak intermolecular forces. With no charged particles that can move, it can’t conduct electricity. Simple molecule are non-polar, so it doesn’t interact with polar water molecules so doesn’t dissolve (HCL and NH3 can form hydrogen bonds though).

Question 22

What is hydrogen bonding?

Answer 22

Type of permanent dipole-dipole interaction. It is found between molecules containing an electronegative atom with a lone pair of electrons, and a hydrogen atom.

Question 23

What is the shape around hydrogen atom in the hydrogen bond?

Answer 23

Linear