In modern terminology, ionization energy is known as ionization enthalpy. The energy required to remove an electron from an atom is known as ionization enthalpy ( IE ). The first ionization enthalpy may be defined as the amount of energy required to remove the most loosely bound electron from the isolated gaseous atom.
In chemistry, it is more normal to give the value, in kJ/mol (or formerly kcal/mol) of the related quantity that (strictly) should be called the "molar ionization potential" but is often just called "ionization potential". Ionization enthalpy is called as ionization potential because it is measured as the amount of potential required to remove the most loosely held electron from the gaseous atom. It is expressed in terms of either kJ/mol or electron Volts/atom.
If a second electron is to be removed from the same element the energy required will be higher than that required for removal of the first electron because it is more difficult to remove an electron from a positively charged species than from a neutral atom.
The ionization potentials increases while moving from left to right in a period. This trend can also be explained in terms of the increase in nuclear charge and decrease in size from left to right in a period. Generally the first ionization enthalpy decreases down a group in the periodic table. As we move down the group, the outer electrons, which are to be removed, are farther from the nucleus and there is an increasing screening of the nuclear charge by the electrons in the inner shells. Consequently the removal of electrons becomes easier down the group.
Ionization enthalpy decreases when the shielding effect of inner electrons increases. This is because when the inner electron shells increases, the attraction between the nucleus and the outermost electron decreases.
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