Assertion A: Generally, 3d transition metals have high melting points. Reason R: Involvement of 3d-electrons in addition to 4s-electrons in the interatomic metallic bonding. Choose the most appropriate answer:

Solution & Explanation

Concept: melting points of transition metals and the role of metallic bonding. Assertion: 3d transition metals generally have high melting points — TRUE. They melt at much higher temperatures than s-block metals. Reason: this is because, in addition to the outer 4s electrons, the (n-1)d, i.e. 3d, electrons also take part in the interatomic metallic bonding — TRUE. Why the reason explains it: the greater the number of unpaired/valence electrons available for delocalised metallic bonding, the stronger the binding between atoms, hence the higher the melting point. Both 4s and 3d electrons contribute, giving strong cohesive forces. (This also explains why metals with roughly half-filled d-shells, like Cr, W, have the highest melting points, while Zn/Cd/Hg with completely filled d-shells, where d-electrons are not available for bonding, melt low.) So A and R are both correct and R is the correct explanation of A — answer (A).