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What is the valency of the mercurous ion?
(A) Two
(B) One
(C) Three

Last updated date: 17th Apr 2024
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Hint: Mercury is a transitional element and has the fairly unique ability of forming polycations i.e., polyatomic ions having positive charge with the mercurous ion being widely found in the compound known as calomel.

Complete step-by-Step Solution:
> Before we answer this question, let us first dive into the fascinating world of the anomalies that are Mercury’s polycations.
> Mercury polycations are polyatomic cations that contain only mercury atoms. The best-known example is the $H{{g}_{2}}^{+2}$ ion, found in mercury(I) (mercurous) compounds. The existence of the metal–metal bond in Hg(I) compounds was established using X-ray studies in 1927 and Raman spectroscopy in 1934 making it one of the earliest, if not the first, metal–metal covalent bonds to be characterised.
> Other mercury polycations are the linear $H{{g}_{3}}^{+2}$ and $H{{g}_{4}}^{+2}$ ions, and the triangular ion and a number of chain and layer polycations.
> Now let us now look into the mercurous ion ($H{{g}_{2}}^{+2}$)in particular.
It is the best known polycation, in which mercury has a formal oxidation state of +1. The $H{{g}_{2}}^{+2}$ ion was perhaps the first metal-metal bonded species confirmed. $H{{g}_{2}}^{+2}$ is stable in aqueous solution, where it is in equilibrium with Hg2+ and elemental Hg, with Hg2+ present at around 0.6%.
This equilibrium is readily shifted by the addition of an anion which forms an insoluble Hg(II) salt, such as S2−, which causes the Hg(I) salt to completely disproportionate, or by the addition of an anion which forms an insoluble Hg(I) salt, such as Cl−, which causes the elemental mercury and Hg2+ to completely recombine into the mercury(I) salt. Known minerals that contain the $H{{g}_{2}}^{+2}$ cation include eglestonite.

With this analysis and data, we can safely conclude that the answer to this question is (b) One.

Note: Do not get confused between the $H{{g}_{2}}^{+2}$ and Hg2+ ions as they are very different kinds of cations. Remember that this ability to form polyatomic ions is a fairly unique ability of Hg and you must be extremely careful to ensure that no misunderstandings are made in the process.