​Open the attached image, which of the following atoms are isotopes?
Atoms 1 & 2 because they have equal numbers of neutrons. The neutron defines the identity of the element.
Atoms 1 & 3 because they have equal numbers of electrons. The electron defines the identity of the element.
Atoms 1 & 3 because they have equal numbers of protons. The proton defines the identity of the element.
Atoms 2 & 3 because they have equal mass numbers. The mass number defines the identity of the element.