The transition metals are the metals which have incompletely filled d orbital in neutral or ionised state . Due do this many unpaired electrons are present in the atoms which lead to strong intermetallic attraction and high heat of atomisation.
Zn, Cd , Hg are said to be non transition metals due to presence of complete d electrons in neutral as well as ionised state. Also because these atoms have completely paired electrons , the intermetallic bonding is weaker. Zn have one of the lowest heat of atomisation with only 126 kj per mole.
earth science 
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