Energy which is required to pull out one mole of the outermost shell's electrons in a neutral atom from one mole of gaseous atoms to produce 1 mole of gaseous ions each with a charge of 1+.
[tex]A\to A^++ e^{-}[/tex] , where A is any neutral atom.
In the periodic table, Its value decreases from top to bottom in groups and increases from left to right across a particular period.
