Electron affinity is the energy released when a neutral atom in the gas phase gains an electron to form a negative ion.
It is measured in KJ.mol– or KJ/mol.
As you move from left to right across a period in the periodic table, the nuclear charge increases while the electron cloud size remains relatively constant. This results in a general trend of increasing electron affinity across a period, meaning that it becomes more favourable for an atom to gain an electron
As you move down a group in the periodic table, the electron cloud size increases while the nuclear charge remains relatively constant. This results in a general trend of decreasing electron affinity down a group, meaning that it becomes less favourable for an atom to gain an electron.
For any given atom, the outermost valence electrons will have lower ionisation energies than the inner-shell kernel electrons. As more electrons are added to a nucleus, the outer electrons become shielded from the nucleus by the inner shell electrons. This is called electron shielding or shielding effect.