Instead, carbon shares its four outer electrons with other atoms in electron-pair bonds, thereby gaining eight electrons around its inner helium core.

The energy required to remove one electron from a gaseous neutral atom and convert it into a positive ion is its first ionization energy (abbreviated IE). Subsequent energies needed to take away more electrons from the positive ion are called the second, third, etc. ionization energies. Ionization energies, like other energies dealing with molecules in chemistry, usually are measured in kilocalories per mole of molecules or atoms.

The ionization energies of a lithium atom give us information about its electron-shell structure. It is relatively easy to take away the lone outer electron in the second shell (opposite), because this electron is far from the nucleus and sees only a net nuclear charge of +1 because of shielding by the inner electrons:



The second ionization energy is much higher, and beyond what can be accomplished in chemical reactions: