Atoms of different kinds combine to form molecules of:

1. an element
2. a compound
3. a mixture
4. all of the above

The compound used as common salt is:

1. sodium chloride
2. calcium chloride
3. sodium oxide
4. hydrogen chloride

The atomicity of ozone is:

1. 2
2. 3
3. 4
4. 8

The molecular formula of ammonia is:

1. NO
2. NH₃
3. NO₂
4. N₂O