Define the following terms —

(a) atomic number and (b) mass number.

(a) Atomic number (Z) — The atomic number of an atom is equal to the number of protons in it's nucleus (which is same as the number of electrons in a neutral atom). i.e.,
Z = number of protons in the nucleus of an atom.

(b) Mass number (A) — The mass number of an atom is equal to the total number of nucleons (i.e., the sum of the number of protons and the number of neutrons ) in it's nucleus. i.e.,
A = number of protons + number of neutrons in the nucleus of an atom.