each successive ionization energy is markedly greater than the last. particularly great increases occur after any given block of atomic orbitals is exhausted of electrons. for this reason, ions tend to form in ways that leave them with full orbital blocks. for example, sodium has one "valence electron" in its outermost shell, so in ionized form it is commonly found with one lost electron, as . on the other side of the periodic table, chlorine has seven valence electrons, so in ionized form it is commonly found with one gained electron, as . caesium has the lowest measured ionization energy of all the elements and helium has the greatest. in general, the ionization energy of metals is much lower than the ionization energy of nonmetals, which is why, in general, metals will lose electrons to form positively charged ions and nonmetals will gain electrons to form negatively charged ions.