Bromine has 35 electrons arranged as 2, 8, 18, 7 across 4 shells, which is why it sits in period 4.
Electrons fill orbitals in order of increasing energy: 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p… Each s orbital holds 2 electrons, p holds 6 and d holds 10. Placing Bromine's 35 electrons in that order gives 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p5.
| Atomic number | 35 |
| Symbol | Br |
| Atomic mass | 79.904 u |
| Group | 17 |
| Period | 4 |
| Block | p-block |
| Category | Halogen |
| Valence electrons | 7 |
| Element | Configuration |
|---|---|
| Fluorine | [He] 2s2 2p5 |
| Chlorine | [Ne] 3s2 3p5 |
| Iodine | [Kr] 4d10 5s2 5p5 |
| Astatine | [Xe] 4f14 5d10 6s2 6p5 |
| Tennessine | [Rn] 5f14 6d10 7s2 7p5 |
Elements in the same group have the same number of outer electrons, which is why they behave alike.