In the modern periodic table, Hydrogen is the first element and also it has only one valence electron in its 1s subshell. Therefore hydrogen exhibits properties of both the alkali metals and the halogens. Hence the position of Hydrogen is not fixed in the periodic table it is placed on the top of Alkali metals and also on the top of the halogens. If hydrogen is considered to be a halogen it requires only one electron to acquire the stable noble gas configuration of helium and if it is considered to be the Alkali metal then hydrogen can lose the only valence electron to form hydrogen ion. The dual nature of hydrogen can be justified by following points:

1. As stated earlier, hydrogen like other alkali metal lose the only electron to form unipositive ion.

3. When hydrogen is present in compound state like HCl it exhibits +1 oxidation state like other alkali metals.

4. Hydrogen has strong affinity towards electronegative elements and combine with them to form compounds.

Resemblance with halogens:

1. Like other halogen elements hydrogen requires only one electron to complete its octet and to achieve stable noble gas configuration.

2. Hydrogen exists as diatomic molecule and also form many covalent bonds to form covalent compounds.

Though hydrogen exhibits properties of both alkali metals and halogens, it does not resemble any of the groups completely. For instance hydrogen does not exhibit any metallic characteristics like other alkali metals and it is also less reactive as compared to halogens. Hence, hydrogen is best placed separately in the periodic table.