Ionization energy is the energy required to remove an electronfrom an atom or ion. Low ionization energy indicates that it takesless energy to remove an electron from the atom. The alkali metalsare in Group I on the Periodic Table. This indicates that theiratoms have only one electron (out of a possible eight) in theiroutermost energy level. Therefore it takes less energy to removethe single outermost electron.Moving across a period on the Periodic Table, ionizationenergies increase because there are more and more electrons in theoutermost energy level, requiring more energy to remove anelectron.