Atomic number: Number of protons present in the nucleus is called atomic number (Z) of an element.
Atomic weight: The sum of mass number of proton and mass number of neutron is called atomic weight or atomic mass number (A).
A = Z + n°
Example: 19K39
Z = 19 and A = 39 as per atomic weight definition.
A = Z + n°
n° = A - Z
n° = 39-19
n° = 20
Similarly for 53I 127, no of neutron n° = A - Z = 127-53 = 74
Isotopes: Atoms of the same element contains different number of neutrons, are called isotopes. They differ by their atomic weight but exhibits same chemical properties. Example – Hydrogen and deuterium.