What do we call different versions of an atom that have the same number of protons but different numbers of neutrons?

The term for different versions of an atom that have the same number of protons but different numbers of neutrons is isotopes. Isotopes of an element share the same atomic number, which is determined by the number of protons, but they differ in mass because of the varying number of neutrons. This variation can affect the stability and nuclear properties of the atom.

For example, carbon has isotopes such as Carbon-12, which has 6 protons and 6 neutrons, and Carbon-14, which has 6 protons and 8 neutrons. While they behave chemically the same way due to having the same number of protons, their physical properties may differ due to the differing number of neutrons.

The other options refer to different concepts: ions are atoms or molecules that have gained or lost one or more electrons, thus acquiring a charge; atoms are the basic units of matter that make up all substances; and molecules are formed when two or more atoms bond together. Each of these terms describes a different aspect of atomic and molecular structure, but they do not specifically refer to the variations of an atom based solely on neutron count.