Reaction of Halogens with metals

Why halogen form ionic bonds with most metals but covlent   bonds with aluminum and beryllium.

Halogens- metal compounds are mostly ionic in nature

 Reason

1)     There is a high electronegativity difference between halogens and metals.

2)     Secondly halogen have higher electron affinity so negative halide ion X^─ formation is easier.

3)     Thirdly Halogens have high positive standard reduction potential (X₂/2X^─) if we convert this reduction potential into energy term by using the following equation  

             ∆G= ─nFEͦ         n = no of electrons (in this case 2)

                                         F = Faraday constant

Eͦ value decrease down the group and the halide ion X^─  formation energy also decreases.

That’s why most of the iodide are partially covalent and have low melting points than the fluorides

There are certain metals like aluminum and beryllium which farm covalent compounds with halogens

 Reason

 Beryllium in Be⁺² Oxidation state and Aluminum in Al⁺³ state have very small size and that's why they form covalent compound with halogens.