Question

Atomicity of an element is _________.

Answer 1

Whenever it is asked about the total number of atoms that are present in a particular compound, the term atomicity comes to play the sole role. If the atomicity of an element, a compound or even a mixture is known, the total number of atoms can be predicted. The knowledge of the total number of atoms comes handy when there is a qualitative analysis of a particular element, compound or even a mixture.

Complete step by step answer:
Atomicity is defined as the total number of atoms that constitute a molecule.

For example, each molecule of oxygen (${O_2}$ ) is composed of two oxygen atoms. So the atomicity of oxygen is 2. In older contexts, the term atomicity is sometimes used in the same sense as valency.
On the basis of atomicity, molecules can be classified as :
Monoatomic - That is composed of only one atom e.g. $He,{\text{ }}Ne,{\text{ }}Ar$ (all noble gases are monatomic)
Diatomic – That is composed of two atoms e.g. ${H_2},{\text{ }}{N_2},{\text{ }}{O_2}$
Triatomic – That is composed of three atoms e.g. ${O_3}$ (ozone, that forms a layer over the troposphere)
Polyatomic – That is composed of three or more than three atoms e.g. ${P_4},\;{S_8}$
In the compounds also, the atomicity can be applied for the same. For example, the atomicity of carbon dioxide ($C{O_2}$ ) is three.

Note:
All 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. Their atomicity cannot be determined and is usually considered as 1. Atomicity may vary in different allotropes of the same element.