Question

Why is the second ionization energy of an element larger than the first ionization energy ?

Answer 1

Second ionisation energy is defined by the equation:

X+ -------> X2+ +e-



It is the energy needed to remove a second electron from each ion in 1 mole of gaseous 1+ ions to give gaseous 2+ ions.
When the first electron is removed from an element,its effective nuclear charge increases. Once,the effective nuclear charge increases,the hold of nucleus over its outermost electrons increases.  So,due to the increase in effective nuclear charge,more energy is required to remove the second electron from the element.

Answer 2

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An element's second ionization energy is the energy required to remove the outermost, or least bound, electron from a 1+ ion of the element. Because positive charge binds electrons more strongly, the second ionization energy of an element is always higher than the first.