The nucleus is a positively charged core located at the centre of the atom. Almost the entire mass of the atom is concentrated in its nucleus.
A nucleus consists mainly of two types of particles , protons and neutrons. Collectively these two particles are sometimes called as nucleons .

(i) Atomic Number: It is the number of protons present in the nucleus. It is also equal to the number of electrons in the neutral atom.

(ii) Mass Number : Total number of protons and neutrons is the mass number. If atomic number of an atom is Z, number of neutrons present in the nucleus of the atom is N, then its mass number can be given by the expression, A = Z + N, where A = mass number.

Depending on the number of protons and neutrons in the nucleus, nuclei may be related to each other in three ways-

(i) Isotopes: Nuclei having same atomic number but different mass number are called isotopes of each other.
Eg : 1H2, 1H3

(ii) Isobars: Nuclei having same mass number but different atomic number are called isobars.
Eg: 6C14 , 7N14

(iii) Isotones: Nuclei having same number of neutrons, but different atomic number and mass number are called isotones.
Eg : 1H3 , 2He4

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