The lowest energy levels which electrons can occupy in an atom, ion, or molecule. For hydrogen and helium, this is the energy level closest to the nucleus. However, this energy level can hold only 2 electrons, so for the next-heaviest element, lithium, the ground state has 2 electrons in the lowest energy level and 1 in the second level. The second energy level can contain a maximum of 8 electrons, the third level a maximum of 18 electrons, and so on (2n² electrons in the nth level). The ground states for the heavier elements may therefore have some of their electrons in quite high energy levels.