This element was known in antiquity. In Latin it was called hydragyrum (liquid silver).

Ionization energies
HgI 10.4 eV, HgII 18.8 eV, HgIII 34.2 eV.

Absorption lines of HgI
The resonance line at 1849(2) is present as a weak absorption line in the sun (McAlliatrer 1960).

Behavior in non-normal stars
HgII was first identified by Bidelman (1962a) to be the element responsible for a strong line at 3984 in the spectra of many Bp stars of the Mn subgroup. Because in general Mn is accompanied by Hg, the group is usually called the Hg-Mn subgroup instead of the former designation of Mn subgroup. In the Bp stars 0.050 < W < 0.300 Å, whereas the line is absent in normal stars. The identification of Hg II was strengthened by the identification of HgI lines (4046, 4358 and 5460 from M.l) by Bidelman (1966) in the Bp and Ap stars. Adelman (1988)measured W(4358) = 0.003 in a Hg-Mn star.

A proposed identification of HgIII is very controversial. For a discussion, see Dworetsky (1985).

Hg has seven stable isotopes, namely Hg 196, 198, 199, 200, 201, 202 and 204. In the solar system their frequencies are respectively 0.2%, 10%, 17%, 23%, 13%, 30% and 7%. There exist 26 unstable isotopes and isomers. Bidelman (1962a) called attention to the fact that the wavelength of the 3984 line does not correspond exactly to the laboratory wavelength, which fact he attributed to the different isotopic composition in different stars. What varies is the proportion of isotopes 198, 200, 202 and 204. Hg204 seems to predominate in late Ap stars, according to Dworetsky (1985). Leckrone et al. (199la, l991b) have recently shown that, in one Ap star of the Hg-Mn subgroup, practically all the Hg is in the form of Hg204.

Hg 199, 200, 201 and 202 can be produced by either the r process or the s process. Hg196 is a pure p process, Hg198 a pure s process and Hg204 a pure r process product.

Published in "The Behavior of Chemical Elements in Stars", Carlos Jaschek and Mercedes Jaschek, 1995, Cambridge University Press.