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COMEDK202510 May 2025Evening ShiftChemistryClassification of Elements and Periodicity in PropertiesActual

The first ionisation enthalpy of Al in kJ mol ⁻¹ is: [Given the first ionisation enthalpy of Na , Mg and Si in kJ mol ⁻¹ are 497, 738 and 787 respectively]

Options

  1. A857
  2. B769
  3. C578
  4. D488

Correct answer

C. 578

Step-by-step solution

The first ionisation enthalpy values for the elements in the third period are as follows: Na (497), Mg (738), Al, and Si (787). The general trend for ionisation enthalpy across a period is an increase due to the increase in effective nuclear charge. However, there are exceptions due to the stability of electronic configurations. The electronic configuration of Mg is [Ne] 3s^2 , which is a fully filled s-orbital, making it exceptionally stable. Consequently, the first ionisation enthalpy of Mg is higher than that of

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