Describe giving reason which one of the following pairs has the property indicated?
(a). `Fe` or `Cu` has higher melting point.
(b). `Co^(2+)` or `Ni^(2+)` has lower magnetic moment.

Explanation

(i) Fe has higher melting point than Cu. This is because Fe has four unpaired electrons in 3d -subshell while Cu has only one electron in the 4s-subshell. Hence, metallic bonds in Fe are much stronger than those in CU.
(ii) `._(27) Co = [Ar] 3d^(7) 4s^(2) ,Co^(2+)= [ Ar] 3d^(7) ( 3 ` unpaired electron)
`. _(28) (Ni) =[Ar] 3d^(8) 4s^(2) , Ni^(2+) = [Ar] 3d^(8) ( 2` unpaired electrons)
Hence, Ni has lower magnetic moment than Co.