Question

What is the atomicity of ${{\text{H}}_{\text{2}}}{{\text{O}}_{\text{2}}}$ ?

Answer 1

In chemistry, the term atomicity is used to denote the total number of atoms which constitute a molecule.
For example, a molecule of oxygen ${{\text{O}}_{\text{2}}}$ is made up of two oxygen atoms and so its atomicity will be 2.

Complete step by step answer:
First, let us understand the different types of atomicity.
On the basis of atomicity, molecules can be classified into the following categories.
Monoatomic molecules: These molecules are composed of only one atom. For example, helium $\left( {{\text{He}}} \right)$ , neon $\left( {{\text{Ne}}} \right)$ , argon $\left( {{\text{Ar}}} \right)$ , krypton $\left( {{\text{Kr}}} \right)$ , xenon $\left( {{\text{Xe}}} \right)$ , radon $\left( {{\text{Rn}}} \right)$ are all monatomic. In fact, all noble gases are monatomic gases as all of them are composed of a single atom only.
Diatomic molecules: These molecules are composed of two atoms. For example, hydrogen $\left( {{{\text{H}}_2}} \right)$ , oxygen $\left( {{{\text{O}}_2}} \right)$ , nitrogen $\left( {{{\text{N}}_2}} \right)$ etc. are all diatomic gases as all of them are composed of two atoms.
Triatomic molecules: These molecules are composed of three atoms. For example, ozone $\left( {{{\text{O}}_3}} \right)$ is a triatomic gas molecule as it is composed of three atoms.
Tetra atomic molecules: These molecules are composed of four atoms. For example, phosphorus $\left( {{{\text{P}}_4}} \right)$ is a tetra atomic molecule as it is composed of four atoms.
Polyatomic molecules: These molecules are composed of more than four atoms. For example, sulphur $\left( {{{\text{S}}_8}} \right)$ is a polyatomic molecule as it is composed of eight atoms.
Now, hydrogen peroxide having the chemical formula ${{\text{H}}_{\text{2}}}{{\text{O}}_{\text{2}}}$ is composed of two hydrogen atoms and two oxygen atoms. Overall, it is composed of four atoms and so ${{\text{H}}_{\text{2}}}{{\text{O}}_{\text{2}}}$ is a tetra atomic molecule and its atomicity is 4.

Note:
All metals as well as some elements do not possess a simple structure but consist of a very large and indefinite number of atoms bonded together. Thus, their atomicity cannot be determined and is generally considered to be 1. For example, carbon has atomicity 1.
Atomicity can also vary in different allotropes of the same element.