If two atoms have the same number of protons but different number of neutrons, they are isotopes.

• A. Isomers
• B. Ions
• C. Isotopes
• D. Elements

Answer: (C)

Isotopes are atoms of the same element that differ in the number of neutrons. Because they share the same number of protons, they have the same atomic number and identity, but the different neutron count changes the mass. Neutrons add to mass without altering the electronic structure much, so isotopes behave chemically very similarly even though some may be more or less stable (or radioactive). For instance, carbon-12 and carbon-14 both have 6 protons; the first has 6 neutrons, the second 8 neutrons. The other ideas don’t fit this situation: isomers involve different arrangements of atoms in a molecule, ions are atoms with a net electric charge due to electron gain or loss, and elements are defined by their proton count.