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Solution


Q.

The correct option with order of melting points of the pairs (Mn, Fe), (Tc, Ru) and (Re, Os) is :   [2025]
1 Mn < Fe, Tc < Ru and Os < Re  
2 Fe < Mn, Ru < Tc and Re < Os  
3 Mn < Fe, Tc < Ru and Re < Os  
4 Fe < Mn, Ru < Tc and Os < Re  

Ans.

(1)

In any row the melting points of these metals rise to a maximum at d5 except for anomalous values of Mn and Tc and fall regularly as the atomic number increases. The maxima at about the middle of each series indicate that one unpaired electron per d orbital is particularly favourable for strong interatomic interaction. In general, greater the number of valence electrons, stronger is the resultant bonding. Because of stable half filled configurations, Mn and Tc have lesser tendency to participate in metallic bonding, hence have lower melting points than expected value.