When they react, both chlorine and bromine need to accept an electron into their outer electron shell to complete the shell and form chloride and bromide ions. There are three factors that affect how easily an electron is accepted:
1. The number of protons in the nucleus. Protons are positively charged and attract negatively charged electrons. A bromine atom has 35 protons but a chlorine atom only has 17. This means that a bromine nucleus has a stronger attraction for an electron than a chlorine nucleus.
2. The atomic radius of the atom. The bromine atom has one more electron shell than the chlorine atom. This makes the radius (the distance from the nucleus to the outer shell) of the bromine atom larger than that of the chlorine atom. The bromine nucleus therefore has to attract an electron from further away, and this greater distance means the electron is attracted less strongly.
3. The shielding within the atom. The number of full electron shells between the nucleus and the electron also affects the strength of the attraction – this is called shielding. Because a bromine atom has one more full shell than a chlorine atom, it has more shielding which means the attraction between the nucleus and an electron is weaker.
Although the bromine nucleus is more positively charged than the chlorine nucleus, the increase in the radius and the extra shielding in the bromine atom outweigh this factor, which means that an electron is more easily attracted into the outer shell of a chlorine atom than that of a bromine atom, so chlorine is more reactive.