Question

Given below are two statements: Statement (I): The first ionisation enthalpy of the elements Na, Mg, Cl and Ar follows the order \[ Na>Mg>Cl>Ar \] Statement (II): Among Ca, Al, Fe and B, the third ionisation enthalpy is very high for Ca. In the light of the above statements, choose the correct answer from the options given below:

• A. Both Statement I and Statement II are true
• B. Both Statement I and Statement II are false
• C. Statement I is true but Statement II is false
• D. Statement I is false but Statement II is true

Answer 1

The Correct Option is D

Concept: Ionisation enthalpy generally increases across a period due to increasing effective nuclear charge.
Step 1:Check Statement I} Across period 3: \[ Na<Mg<Cl<Ar \] Thus the correct increasing order is: \[ Na<Mg<Cl<Ar \] But the statement gives the reverse order. Hence Statement I is false.
Step 2:Check Statement II} Electronic configurations: \[ Ca : [Ar]4s^2 \] After removal of two electrons: \[ Ca^{2+} = [Ar] \] The third electron would be removed from a noble gas configuration. Thus the third ionisation enthalpy is very high. Hence Statement II is true. \[ \boxed{\text{Statement I is false but Statement II is true}} \]