The first row of the periodic table only consists of two elements: hydrogen (H) and helium (He), which have the simplest electrical structures with one and two electrons, respectively.
| Hydrogen | Helium |
| Atomic Number: 1 Chemical Symbol: H Atomic Mass: 1.008 atomic mass units (u) | Atomic Number: 2 Chemical Symbol: He Atomic Mass: 4.0026 atomic mass units (u) |