# Chlorine and Bromine have similar properties. What is the best explanation for this?

Elements in the same group have similar properties because their electron configurations are similar. In particular they have the same number of valence electrons, which are the in the outermost (highest energy) $\text{s}$ and $\text{p}$ orbitals.
In the case of the halogens, the atoms of each element has seven valence electrons, $\text{ns"^2"np"^5}$, where $\text{n}$ represents the energy level. The electron configuration of chlorine is ["Ne"]"3s"^2"3p"^5", and the electron configuration of bromine is ["Ar"]"3d"^10"4s"^2"4p"^5".