Hydrogen is unique among the elements because 1. it has only one valence electron. 2. it is the only element that can emit an atomic spectrum. 3. its electron is not at all shielded from its nucleus. 4. it is the lightest element. 5. it is the only element to exist at room temperature as a diatomic gas.

3,4

Explanation:

Hydrogen has no other electron hence there is no screening of the valence electron by inner electrons. It is the lightest known element with a relative molecular mass of 2. Screening effect refers to the fact that inner or core electrons prevent the outermost electron from feeling the attractive force of the nucleus.