Ionization Potential : Atomic Reactivity :: Electron Affinity : ?

Ionization potential tells how hard it is to pull an electron out of an atom, so a high value means the atom is less eager to lose an electron and therefore less reactive. Electron affinity measures how much energy is released when an atom gains an electron, so a large negative value means the atom strongly wants to accept an electron and is therefore more chemically reactive. Thus the analogy is Ionization Potential : Atomic Reactivity :: Electron Affinity : Chemical Reactivity. For example, fluorine has a very large electron affinity, which explains why it is extremely reactive with metals.