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Ionization energy
(or potential) of an element is defined as “The amount of energy required or needed to remove an electron from the neutral gaseous atom to form unipositive ion , for example Na → Na+ + e– + energy
This is measured in electron volts per atom or kJ mol-1. For example IE of Na is 495 kJ mol-1.
Trends in ionization energy of elements.
1 – Ionization energy increases from left to right in a period. This is because the nuclear charge increases and the electrons are more strongly held and hence more energy is required to remove an electron.
2 – Ionization energy decreases down a group. This is because as we go down a group, the outermost electron (which is being removed) is farther from the nucleus and hence less energy is required to remove it.