Why are hydrogen and oxygen referred to as existing in the diatomic form?

Answer 1

When hydrogen and oxygen exist in gaseous form in nature, they form molecules of two atoms each. That is to say they will form H2 or O2 rather than just existing as H or O. This is also true for nitrogen, fluorine, chlorine, bromine, iodine and astatine.

Answer 2

Hydrogen and oxygen exist in the diatomic form because they tend to form stable molecules by sharing electrons between two atoms of the same element. In their diatomic form, hydrogen exists as (H_2) and oxygen as (O_2). This arrangement allows them to achieve a full outer electron shell, making them more stable.