Question

1. how are the isotopes of an element different? their mass numbers are different. their electron numbers are different. their proton numbers are different. their atomic numbers are different.

Explanation:

Isotopes of an element have the same number of protons (so same atomic number) and in a neutral state, same number of electrons. The difference lies in the number of neutrons. Since mass number = number of protons + number of neutrons, different number of neutrons means different mass numbers.

Answer:

Their mass numbers are different.