Electron Affinity
Electron Affinity: Electron affinity is defined as the energy change that accompanies the addition of an electron to an atom. Some atoms readily attract electrons, and the electron affinity has a negative value, meaning that energy is released. Most atoms, however, do not accept additional electron readily and the electron affinity is a positive value, indicating that energy must be used to add the electrons. For example, the addition of electron to a chlorine atom is accompanied by an energy change of −349 kJ/mol. The negative sign indicates that energy is released during the process.
Cl (g) + e− 
Cl−(g)
ΔE = −349 kJ/mol
Chlorine has the highest affinity for electrons and francium is the lowest. Electron affinity varies diagonally across the periodic table. The atom close to fluorine tends to accept electrons readily, and those close to francium do not. Therefore, in a periodic table electron affinity value decreases down the group, and increases across a period. Down the group, electron affinity decreases because of larger radii.
Electron Affinities in Periodic Table
Exceptions: Among non–metals, however, the elements in the first period have lower electron affinities than the elements below them in their respective groups. Elements with electronic configurations of ns2, ns2 np3 and ns2 np6 have electron affinities less than zero because they are unusually stable. In other words instead of energy being given off, these elements actually require an input of energy in order to gain electrons. Electron affinity values are much smaller than ionization energies. Electron affinity values of element with electronic configurations of ns2 are less than zero because it is a stable, dia–magnetic atom with no unpaired electrons. Electron affinity value of element with electron configurations, ns2 np3 is less than zero because it is a stable atom with 3 unpaired p–orbital electrons each occupying its own sub–shell. Electron affinity value of element with electron configurations, ns2 np6 is also less than zero because it is a stable atom with filled valence (outermost) shell.