Nitrogen has positive electron gain enthalpy whereas oxygen has negative. However, oxygen has lower ionization enthalpy than nitrogen. Explain.

The electron gain enthalpy decreases as we move down a group and increases as we move across a period from left to right. This means it becomes less negative down a group and more negative across a period.

In contrast, the ionization enthalpy increases when moving left to right across a period but decreases when moving down a group.

Now, consider nitrogen (atomic number 7) and oxygen (atomic number 8):

• Nitrogen’s outermost shell has 5 electrons (2s² 2p³).
• Oxygen’s outermost shell has 6 electrons (2s² 2p⁴).

An atom is more stable when it has a fully filled or half-filled orbital. Nitrogen has a half-filled p-orbital (2p³), making it particularly stable. Because of this stability, nitrogen does not easily accept an extra electron, as that would disturb its half-filled configuration. Thus, nitrogen shows a positive electron gain enthalpy and has a high ionization energy, since removing an electron requires significant energy.

Oxygen, on the other hand, has a 2p⁴ configuration. If it loses one electron, it achieves a stable half-filled 2p³ configuration. Therefore, oxygen has a lower ionization enthalpy, meaning it requires less energy to remove an electron. At the same time, oxygen exhibits a negative electron gain enthalpy because it readily accepts an electron to move closer to completing its octet (2p⁶).