Here's why:

* Atomic Number: Bromine's atomic number is 35, meaning it has 35 protons and 35 electrons.

* Electron Configuration: The electron configuration of bromine is 2, 8, 18, 7.

* The first number (2) represents the electrons in the first shell (n=1).

* The second number (8) represents the electrons in the second shell (n=2).

* The third number (18) represents the electrons in the third shell (n=3).

* The fourth number (7) represents the electrons in the fourth shell (n=4).

Therefore, bromine has four electron shells.