In which group should hydrogen be placed? In group 1 or group 17? Why?
Answer»
Hydrogen contains one valence electron in its valence shell and thus, its valency is one. Therefore, hydrogen resembles alkali metals (group 1 elements) as they also contain one electron in their valence shell (alkali metals tend to lose their valence electron).
However, hydrogen also shows similarity with halogens (group 17 elements) as their valency is also one because halogens tend to accept one electron in their valence shell.
Due to this unique behaviour, it is difficult to assign any definite position to hydrogen in the modem periodic table.