General Periodic Trend for Ionization Energy
The first ionization energy (1E1) is the energy required to remove an electron from a neutral atom. The general periodic trend for ionization energy (1E1) is that it increases from left to right in the periodic table and decreases from top to bottom because the atomic radius decreases from left to right and increases from top to bottom.
The general trend is due to the fact that the smaller an atom is, the closer its electrons are to its nucleus, and the harder they are to remove. There are exceptions to the general trend, however, and these exceptions are always associated with the special stability of half-filled and completely filled subshells.