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. A. 1,2,3,4,5.B. 1,4,5.C. 1.2.3.5.D. 3, 4.E. 2, 3, 4, 5.