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The energy released when an electron is added to a neutral gaseous atom is known as electron affinity. Helium has the highest ionization energy in the periodic table while cesium has the lowest ionization energy. The following graph gives decrease of IE in group-1. Ionization energy decreases down the group due increase due to increase in the atomic size (addition of new shell). The following depicts the increase of IE in period. Ionization energy increases across a period due to increase in the nuclear charge. The unit for ionization energy is electron volts or kilo joules per mole. Ionization Energy is the minimum energy required to remove the outermost electron from a gaseous neutral atom to form a cation. The graph below shows decreasing order of atomic radius in period:Ītomic radius increases down the group due to addition of new shell.įollowing graph shows increasing order of atomic radius in group: The atomic radius decreases across a period due to increase in nuclear charge. Atomic radius is expressed in angstrom units. Important periodic properties are:Ītomic radius is the distance from the centre of the nucleus to the valence electron in an energy level. The trend of recurrence of properties is called periodicity. The term periodic properties in elements, refers to the properties that recur at regular intervals.