Here's why:

* Electron configuration of bromine: The electron configuration of bromine is [Ar] 3d¹⁰ 4s² 4p⁵.

* Orbital filling order: Electrons fill orbitals following the Aufbau principle and Hund's rule. This means that electrons first fill the lowest energy levels and then move to higher energy levels.

* 4p orbital: The 4p orbital is the highest energy level that is filled in bromine. It has five electrons, with one unpaired electron.

Therefore, the last orbital filled in a bromine atom is the 4p orbital.