Step 1: Basic Principle
Ionisation energy generally increases across a period, but there is an exception between nitrogen and oxygen.
Step 2: Solution Procedure:
Electronic configurations:
Nitrogen (Z=7): \(1s^2 2s^2 2p^3\) (half-filled p-subshell)
Oxygen (Z=8): \(1s^2 2s^2 2p^4\) (one paired electron in p-subshell)
The half-filled configuration of nitrogen is more stable due to symmetrical distribution and exchange energy. Removing an electron from this stable configuration requires more energy. In oxygen, the electron is removed from a paired orbital, which experiences electron-electron repulsion, making it easier to remove.
Step 3: Required Answer:
The higher ionisation energy of nitrogen is due to the extra stability of its half-filled p-orbitals, option (C).