Ionization Energy

[Steven Garcia 1H]

Why is the ionization energy of oxygen lower than that of nitrogen ?

[Elaine Pham 2E]

Oxygen's ionization energy is lower than Nitrogen's despite the fact that ionization energy typically increases as you go across a period because of their electron configurations. Oxygen's electron configuration is 1s²2s²2p⁴. Nitrogen's electron configuration is 1s²2s²2p³. Because of Nitrogen's 2p³ orbital, Nitrogen has a half-filled orbital that is actually more stable than Oxygen's 2p⁴ orbital. Since Oxygen is less stable, it wants to become more stable by losing that extra electron it has to allow Oxygen to have a half-filled orbital like Nitrogen. Nitrogen, therefore, is more reluctant to have an electron taken away because it is more stable with a half filled orbital. As a result, the ionization energy, or energy needed to take away an electron, of Oxygen is lower than Nitrogen's ionization energy.

[spark99]

This is beacause an atom that has half-filled or full orbitals are more stable. For oxygen, its electron configuration is 1s2 2s2 2p4, and nitrogen's is 1s2 2s2 2p3. Nitrogen has half-filled 2p orbitals, so losing an electron would cause it to become more unstable. However, oxygen can become more stable if it loses on electron, so it has a lower ionization energy compared to Nitrogen.