Question

Is $H_2$ , a monatomic or diatomic gas?

Answer 1

Hint: Molecules consist of atoms of various elements or of the same element. The molecules may contain single atoms to multiple atoms depending on their chemical properties. Hydrogen exists in gaseous form generally, a single hydrogen atom is very rare in nature.

Complete answer:
Monatomic gas is the gas that has molecules containing a single atom only while diatomic gas has the molecule containing two atoms. Similarly, triatomic gas contains molecules with three atoms.
The hydrogen gas exists as a diatomic molecule, as it contains two atoms of hydrogen. The subscript in the atomic formula of hydrogen i.e. $H_2$ represents the presence of two atoms. The existence of a single hydrogen atom is very rare. Although while forming chemical bonds, a single hydrogen atom participates in bond formation, for example, in the case of hydrochloric acid $HCl$ .
In a hydrogen molecule, the two hydrogen atoms containing a single electron form a covalent bond and share the resulting one pair of electrons, thus completing each other’s duplet.
Hence, we can conclude that hydrogen gas is a diatomic gas containing two atoms of hydrogen in a molecule.

Note:
Alone existence of hydrogen atoms is impossible as it is too reactive because of the presence of only one electron. It is very unstable when singly existing so it quickly reacts with other elements and forms a bond.