The amount of energy released when an electron is added to a neutral atom to form an anion is called electron affinity.
- Electron affinity increases going left to right across a period
- Electron affinity decrease going down the group.
Electron affinity = 1/Atomic size
Halogens have high electron affinities due to their small size, high effective nuclear charge and an almost full outer shell of electrons. When an electron is added to halogens with very high electron affinity, high energy is released
The more electronegative atom is fluorine (F), but due to the small size of fluorine when extra electrons are added, repulsion takes place.
In the case of chlorine due to the bigger size electron repulsion is not observed as in the case of fluorine when extra electrons are added, because of the presence of vacant d-orbitals and also electronegativity is not very low. So the electron affinity of chlorine is higher than the fluorine atom.
Chlorine belongs to the 17th group, 3rd period.