Question

What would be the reason for an element to have atoms with different mass numbers

Answer 1

Because different isotopes have different numbers of neutrons, they do not all weigh the same or have the same mass. Different isotopes of the same element have different value of sum of total neutrons and protons.eg chlorine has two isotopes with mass no. 35 and other with mass no.37 and all this happens only because of difference in no. of neutrons .
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Answer 2

Explanation:

Mass of an atom is contributed by mass of protons and mass of neutrons present inside the nucleus of an atom. Atoms of the same element contain the same number of protons but may differ in their number of neutrons. Therefore, an element has atoms with different mass numbers.