Question

Which of the following elements have the highest and the lowest first ionization enthalpy?
(i) Be (ii) B (iii) C (iv) N (v) O

• A. (ii) and (iv)
• B. (i) and (v)
• C. (i) and (iii)
• D. (ii) and (iii)
• E. (i) and (ii)

Hint:

Remember exceptions: - B < Be (p vs s electron) - O < N (pairing effect)

Answer 1

The Correct Option is A

Concept: Ionization enthalpy generally increases across a period, with exceptions due to electronic configuration.

Step 1: General trend.
\[ \text{Be} \text{O} \]

Step 2: Key exceptions.
- B has lower ionization energy than Be (due to p-orbital electron)
- O has lower ionization energy than N (due to electron repulsion)

Step 3: Identify highest and lowest.
- Highest: N (iv)
- Lowest: B (ii)