Question

Why is hydrogen included in the metal activity series?

Answer 1

Hydrogen consists of only one electron. This one electron will be present in $1s$ orbital, as s-orbital can accommodate two electrons, it is deficient of only one electron to get completely filled electronic configuration like halogens. But hydrogen is placed in alkali metals due to the following facts.

Complete answer:
Hydrogen is an element with atomic number $1$ which means the number of protons and the number of electrons is equal to $1$ . This one electron will be present in $1s$ orbital, as an s-orbital can accommodate two electrons. Thus, from the electronic configuration it was clear that hydrogen is deficient of only one electron to get completely filled electronic configuration like halogens. Similar to halogens, it does not react with water very vigorously, so it has the properties similar to halogens. But hydrogen was placed in alkali earth metals.
Alkali earth metals ate the chemical elements that consist of one electron in its valence shell, as hydrogen also consists of one element in its valence shell. It was placed in alkali earth metal. Like alkali earth metals, hydrogen also easily loses one electron in its valence shell and can exist as a positive ion known as cation.

Note:
Halogens are the elements that are only one electron deficient to attain completely filled or stable inert gases configuration. So, they always try to attract electrons from other atoms and have high electronegativity values. But hydrogen does not have a higher electronegativity value similar to alkali earth metals.