Question

give reason Na , K , and Ca metals form hydrides by combination with hydrogen gas but most other metals do not

Answer 1

Na , K , and Ca metals r highly reactive n hence can easily combine with H2 to form their respective oxides which r highly stable .
Others metals r not so reactive as these metals n hence they donot combine with H2 to form their hydrides

Answer 2

Na , K and Ca metals form hydrides by combination with hydrogen gas because these are highly reactive metals and these are react with hydrogen gas , (H2) to form hydrides.

But most other metals do not highly reactive so they do not form hydrides.