Which of the following best describes an isotope?
Structurally variant atoms, which have the same number of neutrons (and protons), but differ in the number of electrons they contain.
Structurally variant atoms, which always have a mass number of 1.
Structurally variant atoms, which have the same number of neutrons (and electrons), but differ in the number of protons they contain.
Structurally variant atoms, which have the same number of protons (and electrons), but differ in the number of neutrons they contain.