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 :
D. Statement I is false but Statement II is true
The first ionisation enthalpy generally increases across a period from left to right due to an increase in effective nuclear charge. For the third period elements Na , Mg , Cl , and Ar , the correct order of first ionisation enthalpy is Ar > Cl > Mg > Na . Thus, Statement I is false. The electronic configuration of Ca is [ Ar ] 4s^2. After the removal of two electrons, Ca ^ 2+ attains the highly stable noble gas configuration of Argon [ Ar ]. Removing a third electron requires breaking this stable noble gas core, which requires a very large amount of energy. Therefore, the third ionisation enthalpy of Ca is exceptionally high compared to Al , Fe , and B . Thus, Statement II is true. Answer: Statement I is false but Statement II is true
Related: Chemistry — Classification of Elements and Periodicity in Properties · All PYQ Banks