Electron Affinity is the amount of energy required for an atom to attain an electron (in kJ/mol) of an atom in its gaseous state.
Ex. X + 1e– → X–
Ex. X– + 1e– → X-2
EA is a measured value determined in units of kJ/mol like ionization energy.
Trend: As you move from:
LEFT → RIGHT EA ↑
TOP → BOTTOM N/A
Why? Right-most elements are nonmetals and like to gain electrons, hence they have an affinity for electrons or ↑ EA.
Left-most elements are metals who have no desire to keep electrons, thus: ↓ EA.