Question

Why is the mass number different in two isotopes?

Answer 1

Answer:

Isotopes of an element differ in the number of neutrons in the nuclei so they have a different mass number as the mass number is the sum of a number of protons and neutrons in the nucleus.

Explanation:

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Answer 2

Answer:

Because they have different numbers of neutrons.

Explanation:

isotopes are that, which have same number of protons but different numbers of neutrons.

and atomic mass = total number of protons and neutrons in an atom.

that's why isotopes have different masses.