Ionization energy is the minimum amount of energy needed to completely remove the valence electrons from the atom/ion and is directly related to the force of attraction on the electrons . The larger the nuclear charge and smaller radii of atom the larger the ionization energy (IE) .
Ionization energy is also more for half filled and fully filled orbitals . There is gradual increase in ionization energy with increasing atomic number. This explains as to why the atomic radius decreases across a period which in turn increases the ionization energy.
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