Isotope is defined as an atom that contains a different number of which particles than protons?

• A. Neutrons
• B. Electrons
• C. Protons and neutrons
• D. Protons and electrons

Answer: (A)

Isotopes are atoms of the same element that differ in the number of neutrons in the nucleus. The number of protons stays the same, which fixes the element’s identity, but adding or removing neutrons changes the mass without changing chemical behavior much. So the particle count that varies relative to protons is neutrons. If you change electrons, you just get ions, not a different isotope. If you change protons, you’d have a different element, not an isotope of the same element.