What distinguishes isotopes of the same element?

• A. They have the same number of protons and electrons, so they share chemical properties; they differ in neutrons and mass.
• B. They have different numbers of protons.
• C. They have different numbers of electrons.
• D. They have different chemical properties.

Answer: (A)

Isotopes are versions of the same element, so they have the same number of protons and the same number of electrons in a neutral atom. That keeps the element’s identity and its chemical behavior the same. What changes is the number of neutrons in the nucleus, which alters the mass but not the charge. So isotopes differ in mass due to different neutron counts, while protons (and electrons) stay the same, keeping chemical properties essentially unchanged. If protons changed, you’d have a different element; if electrons changed, you’d have ions with different charges, not isotopes; and different chemical properties would imply a different electron arrangement, which isn’t the case for isotopes.