Question

Rn why is hydrogen placed first in the periodic table ?

Answer 1

because hydrogen was the first gas found by scientists

Answer 2

Hydrogen can also bond to metal atoms, such as lithium (LiH), calcium (CaH2), etc. In these compounds, the bonding is usually pictured as a metal cation combined with a hydride anion (H-). (On some periodic tables, in fact, hydrogen is placed at the top of Group 7A, since like the halogens, it can form a -1 charge.)