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JEE Main levelSingle correct optionDifficulty 2 of 3pb117

The first ionisation enthalpy of Ga (579 kJ/mol) is slightly higher than that of Al (577 kJ/mol) mainly because

  1. AGa has a larger atomic radius than Al
  2. BGa has a half-filled subshell
  3. CGa has a lower nuclear charge than Al
  4. Dthe poorly shielding electrons of Ga raise the effective nuclear charge felt by the outer electron

Answer: D

Between Al and Ga the ten electrons are added; they shield the nucleus poorly, so increases and the ionisation enthalpy does not fall as expected.
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