The configurations show that the ultimate shells of alkali metals have one s electrons (s1 electron) while the penultimate shell contains eight (s2p6) electrons (Li is exception). When this valence–shell electron is lost, the alkali metals are converted into M+ ions which have inert gas configuration. Since the additional electron enters ns–orbital, these are s–block elements.
Why hydrogen is not placed in IA group even though its electron enters in s orbital ?
Due to its unique properties, the place of hydrogen is not justified along with the alkali metals or with the halogens i.e., group 17 of the periodic table. As it loses one electron to form a unipositive ion, forms oxides, sulfides and halides. But due to high ionization enthalpy (metals will have low ionization enthalpy) it is not considered as a metal. Like halogens it forms a diatomic molecule, a hydride and covalent compounds. But it doesn't resemble the halogens in terms of reactivity as it is very less reactive comparatively. Thus hydrogen has similarities and differences with both alkali metals and halogens.