Final answer:
The covalently bonded mercury (I) ion, or dimercury ion, is stabilized by the inert pair effect, which explains the formation of its stable Hg-Hg bond. Mercury also forms covalent binary halides, making its compounds weak electrolytes in solution.
Step-by-step explanation:
The stability of the covalently bonded mercury (I) ion, also known as the dimercury ion or Hg22+, is related to a phenomenon called the inert pair effect. This effect occurs due to the relatively low energy of the valence s-electron pair in the heavy elements of groups 13, 14, and 15, and also affects mercury (group 12). The inert pair effect leads to the formation of a stable Hg-Hg bond within the ion. Additionally, mercury forms covalent binary halides rather than ionic ones, setting it apart from many other metals.
When mercury (II) chloride, HgCl2, dissolves, most of its molecules remain intact due to their covalent nature, resulting in solutions that are weak electrolytes. This is why for a HgCl2 solution, the current flow is only about 0.2 percent as much as for a NaCl solution. Similarly, understanding the bonding and reactivity of mercury under various conditions, such as with strong oxidizing acids, requires careful attention to its unique chemistry.