What is an isotope?

• A. Isotopes have different numbers of protons.
• B. Isotopes are atoms with different electron configurations.
• C. Isotopes have the same number of protons but different numbers of neutrons.
• D. Isotopes are elements with different atomic numbers.

Answer: (C)

Isotopes are variants of the same element that have the same number of protons but different numbers of neutrons. The proton count fixes the element’s identity on the periodic table, while changing neutrons alters the mass and nuclear properties without changing the element itself. Because the electrons are arranged around the same nucleus, isotopes share most chemical behavior, even though their masses differ. A classic example is carbon: both carbon-12 and carbon-14 have 6 protons, but they have 6 and 8 neutrons respectively, giving different masses and nuclear characteristics.

If protons were different, you’d have a different element, not an isotope. If electron configurations changed, that would reflect a different electron count or arrangement, not just a neutron change. And different atomic numbers mean different elements entirely.