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Isotopes are defined as atoms of the same element having the same atomic number but different mass numbers.
In other words, isotopes have the same number of protons but different number of neutrons.

Take 126 C, 136 C, 146 C. Here, the mass numbers are different but the atomic numbers are same.
126 C has 6 protons and 6 neutrons
136 C has 6 protons and 7 neutrons
146 C has 6 protons and 8 neutrons

The other examples for isotopes are as follows:
(i) Hydrogen (11 H), deuterium (21 H) and tritium (31 H)
(ii) Uranium-235 (23592 U) and uranium-238 (23892 U)

(iii) Chlorine-35 (3517 Cl) and chlorine-37 (3717 Cl)

  • Isobars are atoms of different elements having the same mass number but different atomic numbers. In other words, isobars have different number of protons, neutrons and electrons, but the sum of protons and neutrons (i.e. the number of nucleons) is the same.
    For example, 4018 Ar, 4020 Ca
  • Isotones are atoms of different elements with the same number of neutrons but different mass numbers. In other words, isotones have different number of protons but same number of neutrons.
    For example, 136 C, 147 N
Both these nuclides have seven neutrons.

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