It exists at the centre of an atom, containing entire positive charge and almost whole of mass. The electron revolve around the nucleus to form an atom. The nucleus consists of protons (+ve charge) and neutrons.

(i) A proton has positive charge equal in magnitude to that of an electron
(+1.6 x 10–19 C) and a mass equal to 1840 times that of an electron.

(ii) A neutron has no charge and mass is approximately equal to that of proton.

(iii) The number of protons in a nucleus of an atom is called as the atomic number (Z) of that atom. The number of protons plus neutrons (called as Nucleons) in a nucleus of an atom is called as mass number (A) of that atom.

(iv) A particular set of nucleons forming an atom is called as nuclide. It is represented as ZXA.

(v) The nuclides having same number of protons (Z), but different number of nucleons (A) are called as isotopes.

(vi) The nuclide having same number of nucleons (A), but different number of protons (Z) are called as isobars.

(vii) The nuclide having same number of neutrons (A – Z) are called as isotones.

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