Amount of energy required to remove the highest-energy electron from an isolated neutral atom in the gaseous state. There is an obvious periodicity to the data, with maximum values for the Element occurring in the right-most column of the periodic table; also called "inert" gases. In these gases, the outer electron shell is completely filled, making them very unreactive. elements and minimum values for the alkali metals. All values are positive, indicating that energy is always required to remove an electron from an atom.
Image source: http://en.wikipedia.org/wiki/File:Ionization_energies.png.