It is called subscript indicating the number of atoms.
Explanation:
The chemical compound is formed by two or more elements.
The compound is formed by the sharing, accepting or donating the electrons.
The compound formed by two elements will have different valencies of cations and anions.
In chemical formula of compound specific number of atoms of each element are present.
eg: H2O (2 atoms of hydrogen bonded with one atom of oxygen)
NaCl (1 atom of sodium and one atom of chloriine are bonded.)
NaHCO3 ( 1 atom of sodium, 1 atom of hydrogen, 1 atom of carbon and 3 atoms of oxygen.)
