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Ionization Enthalpy of elements is the amount of energy that an isolated gaseous atom requires to lose an electron in its ground state. The ionization enthalpy increases with increase in nuclear charge. With increase in nuclear charge, the electrons of the outer shell are more firmly held by the nucleus and thus greater energy is required to pull out an electron from the atom. Ionisation enthalpy decreases with increase in atomic size. The first ionization energy of element A is the energy required by an atom to form A + ions.  The unit of ionization energy is given as KJ mol -1 . In the same way, the second ionization energy is nothing but the energy that you need to remove the second electron from its valence shell.

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