Question

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

Answer 1

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.