Question

Why does Hydrogen come in reactivity series even when it is not a metal?

Answer 1

Hydrogen has only 1 electron in its 1s orbital, so its electronic structure closely resembles that of the other alkali metals, which have a single valence electron in a 2s,3s,4s... orbital.You might argue that hydrogen is missing only one electron from having a complete valence shell and that it should be listed in Group VII with the halogen atoms (F, Cl, Br, etc.). This would be valid, too.
However, halogens are extremely electronegative, with F leading the way, and H is not very electronegative, so its chemical properties more closely resemble the alkali metals than the halogens, even though it does not form a true metal (except perhaps at extremely high pressures).That's why the element hydrogen occupies a place in Group I rather than Group VII in the Periodic Table of Elements.

Hydrogen also has a standard electrode reduction potential (Eo) of zero. It is placed in the activity series of metals as a comparison point. (Most reactive on left) Generally, to the left of hydrogen metals have negative reduction potentials, meaning they are more stable as ions. And generally, to the right of hydrogen metals have positive reduction potentials, meaning they are more stable as metal atoms.

Hence, Hydrogen is placed in the Metal Reactivity series