Because mercury (234 K), Cd (592 K)  and Zn (693K)  all have 10 d-electrons and 2 s-electrons. Both the s- and d blocks are filled and bond strength is at a minimum..
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they come under Group 12
 It includes zinc (Zn),  (234 K)
cadmium (Cd)  (592 K) 
and mercury (Hg) (693K) 
they are highly volatile metals volatile metals
the bonding between the atoms is very less therefore they have low melting and boiling point
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