Question

The ionisation energy of nitrogen is larger than that of oxygen because

• A. the size of nitrogen atom is smaller
• B. there is greater attraction of electrons towards the nucleus
• C. half-filled p-orbitals of nitrogen have extra stability
• D. None of the above

Hint:

Similarly, Be has higher IE than B (filled 2s), and Mg has higher IE than Al (filled 3s).

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
Ionisation energy generally increases across a period, but there is an exception between nitrogen and oxygen.
Step 2: Detailed Explanation:
Electronic configurations: \begin{itemize} \item Nitrogen (Z=7): \(1s^2 2s^2 2p^3\) (half-filled p-subshell) \item Oxygen (Z=8): \(1s^2 2s^2 2p^4\) (one paired electron in p-subshell) \end{itemize} 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: Final Answer:
The higher ionisation energy of nitrogen is due to the extra stability of its half-filled p-orbitals, option (C).