Here's why:

* Valency refers to the number of electrons an atom can gain, lose, or share to form chemical bonds.

* Bromine is in Group 17 (also known as Group VIIa) of the periodic table, which are called the halogens.

* Halogens have 7 electrons in their outermost shell. They tend to gain one electron to achieve a stable octet (8 electrons) in their outermost shell.

* Since bromine needs to gain one electron to become stable, its valency is 1.

However, it's important to note that bromine can also form multiple bonds in some compounds. For example, in bromine monofluoride (BrF), bromine has a valency of 1. However, in bromine pentafluoride (BrF5), bromine has a valency of 5. This is because bromine can expand its octet and accommodate more electrons in its outermost shell.