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Classification of Molecules

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notes

The molecules of an element are constituted by the same type of atoms. Molecules of many elements, such as argon (Ar), helium (He) etc. are made up of only one atom of that element. But this is not the case with most of the nonmetals. For example, a molecule of oxygen consists of two atoms of oxygen and hence it is known as a diatomic molecule, O2. If 3 atoms of oxygen unite into a molecule, instead of the usual 2, we get ozone. The number of atoms constituting a  molecule is known as its atomicity.

Metals and some other elements, such as carbon, do not have a simple structure but consist of a very large and indefinite number of atoms bonded together. Let us look at the atomicity of some non-metals.

Atomicity of some elements : 

Type of Element

Name

Atomicity

Non-Metal

Argon

Monoatomic

Helium

Monoatomic

Oxygen

Diatomic

Hydrogen

Diatomic

Nitrogen

Diatomic

Chlorine

Diatomic

Phosphorus

Tetra-atomic

notes

Atoms of different elements join together in definite proportions to form molecules of compounds.

For example, a molecule of water consists of two atoms of hydrogen and one atom of oxygen to form a triatomic molecule H2O.

Molecules of some compounds : 

Compound

Combining Elements

Ratio by Mass

Water (H2O)

Hydrogen, Oxygen

1:8

Ammonia (NH3)

Nitrogen, Hydrogen

14:3

Carbon dioxide (CO2)

Carbon, Oxygen

3:8

notes

Molecule of Element:

The molecules of an element are constituted by the same type of atoms.

Atomicity

The number of atoms constituting a molecule is known as its atomicity. We know that hydrogen exists as H2 in a free state. So, hydrogen’s atomicity would be 2.

Example:

Element Atomicity
Helium Monoatomic
Oxygen Diatomic
Chlorine Diatomic
Phosphorus Tetra-atomic
Sulphur Poly-atomic
Sodium Monoatomic
Copper Monoatomic

notes

Molecules of Compounds:

Atoms of different elements join together in definite proportions to form molecules of compounds.

For example, the ratio by a number of atoms for water is H: O = 2: 1 hence giving rise to formula H2O.

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