Question

Why HYDROGEN is considered as a unique element?

or

Why Hydrogen is never considered as an alkali metal?

Answer 1

Hydrogen is considered as a very unique element.Its valence shell electronic configuration resembles to alkali metals but its most of the properties are similar to non-metals.It is never considered as alkali metals because alkali metals are characterised by their electropositive nature, physical state(most are the solids) and typical metallic behaviour.
Moreover Hydrogen form hydrides with alkali metals as similar to halides.Like NaH and NaCl.Hydrogen also resembles halogens in many ways. Like halogens, hydrogen is a gas, and exists as a diatomic molecule (H ₂ ). It forms covalent compounds like halogens unlike alkali metals.