Atomic terms a) Atomic Number (Z): The atomic number of an element is the number of protons contained in the nucleus of the atom of that element. b) Nucleons: Protons and neutrons are present in a nucleus. So, these fundamental particles are collectively known as nucleons. c) Mass Number (A): The total number of protons and neutrons i.e,  the number of nucleons present in the nucleus is called the mass number of the element. d) Nuclide: Various species of atoms in general. A nuclide has specific values of atomic number and mass number. IUPAC notation of an atom (nuclide) Let X be the symbol of the element. Its atomic number be Z and mass number be A. Then  the element can be represented as   e) Isotopes:   Atoms of the element with same atomic number but different mass number e.g. 1 H 1 , 1 H 2 , 1 H 3 . There are three isotopes of hydrogen. f) Isobars: Atoms having the same mass number but different atomic numbers, e.g. 15 P 32  and 16 S 32  are called isobars. g)