Each mole of bromine atoms must accept one mole of electrons to form one mole of bromide ions (Br-). Bromine gas is diatomic, which means that each mole of bromine gas contains two moles of bromine atoms. Therefore, in order to reduce one mole of bromine gas to form bromide ions, two moles of electrons must be added.
The energy required or released in this process is known as the electron affinity.
