Some ionization potential variations and relationships

Abstract

Abstract Ionization potential variations and relationships are considered in three parts. The first is devoted to an examination of variation of successive ionization potentials involving valence electrons in several elements. With the exception of some Group II elements in particular, the relationship I n =KZ is quite closely approximated in several elements and holds both for s and p electrons, but distinctly not for d electrons ( I n = n th ionization potential; that is, the first ionization potential when Z , the ionic charge = 1, the second when Z = 2, etc.; K is a constant). I n is elevated (i) when passing from p and s electrons in nontransitional elements and (ii) when passing from s to d electrons in transitional elements. The magnitude of Δ I (the difference between two successive ionization potentials) can usually be correlated with the existence of closed shells, subshells, and valence states in general, and may serve as a useful semi-quantitative guide in this respect, provided some allowance is made for properties such as weight and size. Variation of I n in isoelectronic sequences is considered in the second part; the relationship I n = aZ 2 + bZ is closely obeyed in several sequences. For s electron sequences, constant a varies from R (=13·59) in the point-charge sequence H + → N 7+ to ∼0·90 in some 5 s sequences; b varies from 2·2 (Li + → F 7+ ) to 10·9 (He + → N 6+ ). a remains uniform at 3·5 in a series of 2 p sequences (B + → Na 7+ …Ne + → Si 5+ ), whereas b increases from 5·4 to 16·6. The relationship of a third variable, ionic radius ( r ), to I and Z is considered in the third part. r varies smoothly with both I and Z in isoelectronic sequences. Various other regularities may be observed; some involve entry of 3 d and 4 f electrons. For the sequence Na + → Cl 7+ and for Na + K + Rb + Cs + , and Mg 2+ Ca 2+ Sr 2+ Ba 2+ , the relationship I n r = 5Z 1.27 holds fairly well. The quantity 5Z 1.27 I n r , which equals unity ± a couple of per cent for the above cations, has been suggested as a comparative measure of the shielding (screening) efficiency of cations.