There are three competing factors at play here: 1. The effective nuclear charge 2. The size of the atom and the force of attraction according to Coulomb's law 3. A pair of electrons in an orbital The first ionization energy is the energy required to completely remove the first electron from the atom. The higher in energy an electron is to start with, the less additinal energy will be required to remove it, which translates to a lower ionization energy. By the same token, an electron arrangement which is lower in energy will require more energy to remove an electron.

These three factors are required for ionization potential or ionization energy.

Explanation:

Ionization potential refers to the amount of energy which is required for the removal of outermost electron of the atom. If the atom size is big so the outermost electron is far from the nucleus and low energy is required for its removal due to lower force of attraction between nucleus and outermost electron. If the nuclear charge is higher, so the electron is tightly held by the nucleus and require more energy for its removal. Nuclear charge means number of protons present in the nucleus.