Question

Which is the element that has the highest first ionization potential?

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Answer 1

Nitrogen is the element that has the highest first ionization potential.

Answer 2

The Ionization Energy increases across a period, therefore the I.E of O should be highest, but the first I.E of N is greater than O because in the oxygen case the electron being removed is one of the 2px2

pair. The repulsion between the two electrons in the same orbital means that the electron is easier to remove than it would otherwise be. Therefore, the I.E of N is highest.