Atomic Spectroscopy - Sequences

14.   Sequences

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.

• Isoelectronic Sequence

  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.

• Isoionic, Isonuclear, and Homologous Sequences

  An isoionic sequence comprises atoms or ions of different elements having the same charge. Such sequences have probably been most useful along the d- and f-shell rows of the periodic table. Isoionic analyses have also been carried out along p-shell rows, however, and a fine-structure regularity covering spectra of the p-shell atoms throughout the periodic table is known [19].

  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.