Several types of sequences of elements and/or ionization stages are useful because of regularities in the progressive values of parameters relating to structure and other properties along the sequences. All sequence names may refer either to the atoms and/or ions of the sequence or to their spectra.
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 isoelectronic 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 np 2 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.