Question

for atoms to be different isotopes of an element, they must have different numbers of ______
protons
neutrons
electrons
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Explanation:

Isotopes of an element have the same number of protons (which defines the element) but different numbers of neutrons. Protons determine the element, so same element isotopes can't differ in protons. Electrons can change in ions, not isotopes. So isotopes differ in neutrons.

Answer:

B. neutrons