Periodic table group classification
Bromine is located in Group 17 (VIIA) of the periodic table. For representative (main-group) elements, the number of valence electrons is identical to the ones digit of the group number. Elements in this column, known as the halogens, all share the characteristic of having seven electrons in their highest-energy shell.
Electron configuration and energy levels
The total electron count of 35 is distributed across energy levels according to the Aufbau principle. The ground-state configuration is:
\[ \text{[Ar] } 3d^{10} 4s^2 4p^5 \]Valence electrons are defined as the electrons occupying the highest principal energy level (\( n \)). For bromine, the highest level is \( n = 4 \). The electrons in the \( 4s \) and \( 4p \) subshells are the valence electrons:
\[ 4s^2 + 4p^5 = 7 \text{ electrons} \]The \( 3d^{10} \) electrons are part of the inner core because they belong to the \( n = 3 \) level, which is lower in primary energy than the valence shell.
| Category | Subshells | Electron Count | Description |
|---|---|---|---|
| Valence Shell | \( 4s^2, 4p^5 \) | 7 | Active in chemical bonding |
| Core Shells | \( 1s^2, 2s^2, 2p^6, 3s^2, 3p^6, 3d^{10} \) | 28 | Stable, internal electron shells |
| Total | — | 35 | Atomic number of Bromine |
Visualization of valence structure
Chemical reactivity and bonding
The presence of seven valence electrons makes bromine highly electronegative. To achieve a stable octet configuration (\( ns^2 np^6 \)), bromine typically gains one electron to form the bromide ion (Br−). This single electron gap also explains why elemental bromine exists as a diatomic molecule (\( \text{Br}_2 \)), where two atoms share a pair of electrons to mutually complete their valence shells.