Hydrogen is unique among the elements because ________. 1. It is not really a member of any particular group. 2. Its electron is
not at all shielded from its nucleus. 3. It is the lightest element. 4. It is the only element to exist at room temperature as a diatomic gas. 5. It exhibits some chemical properties similar to those of groups 1A and 7A.
Hydrogen can't really be classified into any group in the periodic table because it doesn't have properties that match that of any group exactly. However it exhibits a property of group 1A (formation of a univalent positive ion) and that of group 7A (formation if a univalent negative ion). It contains only one electron, hence this outermost electron is not shielded at all from the nucleus.
