2.3 Molecular Compounds

2.3.1. Bonding and Properties

LO: I understand how the bonding present affects the physical properties of a molecular substance.

In covalent molecular substances the covalent bonds are confined to the individual molecules.

In covalent molecules like CF4:

• hold the atoms together.

• , hold the molecules together.

The physical properties depend on the , forces and not the covalent bonds.

LO: I understand how the bonding present affects the physical properties of a molecular substance.

Covalent bonds

Weak van der Waals forces

intermolecular

2.3.2 Effect of Increasing Atomic Size

LO: I understand how the atomic size affects the physical properties of carbon tetrahalides.

Melting point vs covalent radius of halogen:

As the size increases the melting point .

Covalent Radius (picometres)

MeltingPoint(˚C)

CF4

CCl4

CBr4

CCI4

-200

-150

-100

-50

0

50

100

150

200

50 100 150

This is because the increase in strength with atomic size.

LO: I understand how the atomic size affects the physical properties of carbon tetrahalides.

increases

van der Waals forces

2.3.3 Effect of Increasing the Number of Atoms

LO: I understand how the number of atoms affects the physical properties of the alkanes.

Boiling point of the alkanes:

As the number of atoms increases the boiling point .

This is because the are increasing in number.

LO: I understand how the number of atoms affects the physical properties of the alkanes.

BoilingPoint(˚C)

Number of carbon atomsC3H8

C4H10

C5H12

C6H14

C7H16

C8H18

-50

0

50

100

150

2 4 6 8

increases

van der Waals forces