The ionization energy or ionization potential is the energy required to remove an electron from an atom.

The order of ionization energy from highest to lowest is as follows:

                                  Kr > Br > Se > As > Ge > Ga > Ca > K

When you are looking at a period (row) on the periodic table, as the atomic number increases, the number of protons increases making an increased nuclear force of attraction to electrons, and the number of electrons in each shell increases, filling up electron shells and making them more likely to acquire an electron or electrons to fill the shell rather than give one up. This leads to an increase in ionization energy as you move from left to right across a period.

When you look down a group (column), ionization energy, in general, decreases with increases in atomic size as you move down the group.

There are some exceptions to these generalizations.