Q.Explain the diagonal relationship.

The first element of every group behaves somewhat differently from the rest of its group -- e.g. Li and Be form comparatively more covalent compounds than the rest of the alkali/alkaline-earth metals, which are predominantly ionic. The root cause for the second period specifically is that it has only four valence orbitals available (2s and three 2p, no d orbitals), capping its maximum covalence at 4 -- so boron can only form $BF_4^-$ (covalence 4) while aluminium, one period down with 3d orbitals available, forms $AlF_6^{3-}$ (covalence 6). …