What does the number below each element’s symbol represent

The number written below element's symbol represents the atomic number of that element

Explanation:

The general isotopic representation of an atom is:

where,

Z = Atomic number of the atom

A = Mass number of the atom

X = Symbol of the atom

For Example: Isotope of chlorine (Cl-37)

Isotopic representation is

where,

17 is the atomic number of chlorine

37 is the mass number of chlorine

Cl is the symbol of chlorine element

Hence, the number written below element's symbol represents the atomic number of that element