The atoms of same element which have same number of protons but different no.of neutrons are called isotopes

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Those elements having the same atomic number,
but have different mass numbers, (they differ in the mass number, because they differ in the number of neutrons),
were termed as isotopes.
For eg,
It's isotopes are,
Cl35, and Cl37.
They differ in the atomic weights, but their atomic numbers are the same.

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