When alkali metals are heated with hydrogen they form--------hydrides

yes.

Ionic

When alkali metals are heated with hydrogen, they form metallic hydrides. To understand why this happens, we need to consider the properties of alkali metals.

Alkali metals are highly reactive elements located in Group 1 of the periodic table. They have a single valence electron in their outermost shell, which makes them highly likely to lose that electron and form a positive ion. When alkali metals react with hydrogen, they donate their valence electron to hydrogen atoms, which allows hydrogen to gain an electron and form a hydride ion (H-).

This reaction between alkali metals and hydrogen is highly exothermic, meaning that it releases a significant amount of heat. The heat released is a result of the strong electrostatic attraction between the positively charged alkali metal ion and the negatively charged hydride ion.

The formation of metallic hydrides is a significant characteristic of alkali metals. For example, when sodium (Na) reacts with hydrogen (H2), it forms sodium hydride (NaH). Similarly, other alkali metals like lithium (Li), potassium (K), rubidium (Rb), and cesium (Cs) also form hydrides when heated with hydrogen.

In summary, when alkali metals are heated with hydrogen, they form metallic hydrides due to the transfer of a valence electron from the alkali metal to the hydrogen atom, resulting in the formation of a hydride ion.