Question

Correct formula of calomel is ………………. .

Answer 1

Hint: Calomel is a mineral of mercury deposits. The name is derived from Greek language in which kalo means beautiful and melos means black.

Complete step by step answer:
Calomel is the mineral of mercury chloride. Mercury is an element of the periodic table with atomic number \[80\]. Chlorine is the element of a periodic table with atomic number \[17\]. The electronic configuration of mercury and chlorine are as follows:
Mercury \[Hg = \left[ {Xe} \right]4{f^{14}}5{d^{10}}6{s^2}\]
Chlorine \[Cl = \left[ {Ne} \right]3{s^2}3{p^5}\]
We can assume the formula of mercury chloride can be \[HgCl\] or \[H{g_2}C{l_2}\]. This can be explained by considering the concept of hybridized molecular orbital of mercury.
The outermost shell of mercury is \[6\] having two electrons in the \[6s\] orbital. The outermost shell of chlorine is \[3\] having \[7\] electrons. So to gain the octet the mercury atom has to lose on two electrons and for chlorine to gain octet it has to gain one electron.
On the other hand \[H{g^{2 + }}\] is very stable due to the noble gas electron configuration (a filled valence \[5d\] shell) whereas \[H{g^ + }\] on the other hand has a \[5{d^{10}}6{s^1}\] configuration.
So \[H{g^ + }\] ion will be desperate to lose the electron present in s-orbital. This is possible if two \[H{g^ + }\] ions next to each other are present sparing the s electron. Thus they can combine with each other forming a bond through the hybridized s-orbital. The resulting molecular bonding orbital will have lower than the \[6s\] orbital before. Now both \[H{g^ + }\] ions have a filled outer shell i.e \[6s\] (\[2\] electrons).
So the correct formula of calomel or mercury chloride is \[H{g_2}C{l_2}\].


Note: Mercury chloride involves the bond between two same metal atoms. It has no single electrons and hence shows diamagnetic properties. Calomel is known to be used as a calomel electrode in electrochemistry to measure \[pH\] of solutions.