Why does Zn have a weak metallic bond compared to Chromium although both have a completely filled d orbital?

Answer 1

First, let's get the electron configuration of both elements:

Chromium: [Ar]3d⁵4s¹

Zinc: [Ar]3d¹⁰4s²

As we can see, the valencies of each element is quite different (both do not have a completely filled d-shell: chromium is only half filled for d). Chromium has a free electron in the valence s-orbital, and its d orbital has one "spin up" electron in each sub-shell. The electrons have the same spin angular momentum (spin up), and therefore respond well to EM fields - including other atoms with charge. Chromium will form stronger metallic bonds for this reasons.

Zinc has a pseudo noble gas configuration - since all of the orbitals are filled. The s-orbital also has a spin up electron and a spin down electron - which is all that is allowed for each sub-shell (Pauli's exclusion principle). For these reasons, zinc reacts relatively weakly in comparison to chromium.

I hope this helps.