Elements with low first ionization energies and positive or slightly negative electron affinities, generally form cations. Metals are less likely to gain electrons, but can easily lose their valence electrons to form cations. This means the ionization energy, which is the energy required to remove loosely bound electrons is low. For a metal to gain an electron, an endothermic reaction occurs, and since the pull by the nucleus on electrons is low, it means metals have a low electron affinity.
