Question

Correct order of first ionisation energy of B, In, Ga, and Al is:

• A. \(\mathrm{B}>\mathrm{Al}>\mathrm{Ga}>\mathrm{In}\)
• B. \(\mathrm{B}>\mathrm{Ga}>\mathrm{Al}>\mathrm{In}\)
• C. \(\mathrm{B}>\mathrm{In}>\mathrm{Ga}>\mathrm{Al}\)
• D. \(\mathrm{Al}>\mathrm{B}>\mathrm{Ga}>\mathrm{In}\)

Hint:

For group 13 ionisation energies:
- Decreases down the group due to increasing size.
- Ga>Al due to poor shielding by \(3d\) electrons.
- Compare electron configurations and effective nuclear charge.

Answer 1

The Correct Option is B

The first ionisation energy is the energy required to remove the outermost electron from a neutral atom. For group 13 elements (B, Al, Ga, In), ionisation energy generally decreases down the group due to increasing atomic size, but anomalies occur due to electron configurations.
Step 1: Electron configurations
- B (atomic number 5): \([He] 2s^2 2p^1\)
- Al (13): \([Ne] 3s^2 3p^1\)
- Ga (31): \([Ar] 3d^{10} 4s^2 4p^1\)
- In (49): \([Kr] 4d^{10} 5s^2 5p^1\)
Step 2: Group 13 trends
- Atomic size: Increases down the group (B<Al<Ga<In), reducing ionisation energy. - Electron removed: All have \(ns^2 np^1\), so the electron is removed from the \(np^1\) orbital.
- Anomaly: Ga’s filled \(3d^{10}\) subshell provides poor shielding, increasing effective nuclear charge, so Ga has higher ionisation energy than Al.
Step 3: Ionisation energies
- B: ~801 kJ/mol (smallest, highest ionisation energy). - Al: ~577 kJ/mol. - Ga: ~579 kJ/mol (slightly higher than Al due to \(3d^{10}\)). - In: ~558 kJ/mol (largest, lowest ionisation energy).
Step 4: Order
Highest to lowest: \[ \mathrm{B} (801)>\mathrm{Ga} (579)>\mathrm{Al} (577)>\mathrm{In} (558) \] This matches option (B).