First Ionizing Energy (kJ/mol)
In relation to the formation of ions, a certain amount of energy is needed to completely remove an electron from a gaseous atom or ion and this required energy is known as your Ionization Energy. The First Ionization Energy is the energy needed to remove one or the first valence electron while the Second Ionization Energy is the needed to remove a second valence electron and so on.
In general, ionization energies increase as you go from left to right in a period and decrease as you down a group from top to bottom. The reason behind this is on the amount of electron shielding the valence electron has. If the valence electron is less shielded, it means it is more attracted to the positive nucleus and this means a greater amount of energy is needed to expel such valence electron. The opposite is true for a more shielded valence electron.