Which term best describes variants of the same element that differ in the number of neutrons?

• A. Nuclide
• B. Atomic Number
• C. Isotope
• D. Atomic Mass Unit

Answer: (C)

Isotopes are variants of the same element that differ in neutrons. They all have the same number of protons, which is what defines the element, but the neutron count changes, giving different mass numbers. Because the electron arrangement—hence chemical behavior—depends mainly on the proton (and thus the atomic number), isotopes behave very similarly in reactions. The differences show up more in physical properties like mass and sometimes stability: some isotopes are stable, while others are radioactive and decay at different rates. For example, carbon-12 and carbon-14 are both carbon, so they react the same chemically, but carbon-14 has more neutrons and is radioactive.

Nuclide describes a specific nucleus with a given number of protons and neutrons, so it’s a broader term that could apply to any particular nucleus, whereas isotopes specifically refer to variants of the same element with different neutron numbers. Atomic mass unit is simply a unit of mass, not a label for nuclear variants. Atomic number is the number of protons and remains the same for isotopes of an element, so it doesn’t describe the neutron-based differences.