The first ionisation enthalpy values of third period element, $\mathrm{Na}, \mathrm{Mg}$ an $\mathrm{Si}$ are respectively $496,737$ and $786 \mathrm{~kJ} \mathrm{~mol}^{-1}$. The first $\Delta_\mathrm{i} \mathrm{H}$ value for $\mathrm{Al}$ will be close to

$760 \mathrm{~kJ} \mathrm{~mol}^{-1}$

$950 \mathrm{~kJ} \mathrm{~mol}^{-1}$

$880 \mathrm{~kJ} \mathrm{~mol}^{-1}$

JEE CHEMISTRY MCQ: CLASSIFICATION OF ELEMENTS AND PERIODICITY IN PROPERTIES

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Understanding +4 | -1

The first ionisation enthalpy values of third period element,

\mathrm{Na}, \mathrm{Mg}

\mathrm{Si}

are respectively

496,737

and

786 \mathrm{~kJ} \mathrm{~mol}^{-1}

. The first

\Delta_\mathrm{i} \mathrm{H}

value for

\mathrm{Al}

will be close to

View answer and explanation

Correct option: B

575 \mathrm{~kJ} \mathrm{~mol}^{-1}

Option B (

575 \text{{ kJ mol}}^{-1}

) is correct because aluminum's first ionization enthalpy is lower than magnesium's due to effective shielding of the 3p electron.