A neutral atom and those ions of other elements having the same number
of electrons as the atom comprise an isoelectronic sequence. (Note
that a negative ion having this number of electrons is a member of the
sequence.) An isoelectronic sequence is named according to its neutral
member; for example, the Na I isolectronic sequence.
The atom and successive ions of a particular element comprise the isonuclear sequence for that element.
The elements of a particular column and subgroup in the periodic table are homologous. Thus the C, Si, Ge, Sn, and Pb atoms belong to a homologous sequence having np2 ground configurations (see Table of Ground Levels and Ionization Energies for the Neutral Atoms). The singly ionized atoms of these elements comprise another example of a homologous sequence.