Question

The first ionisation enthalpies of $Na,Mg,Al$ and $Si$ are in the order:

• A. $Na<Mg<Al<Si$
• B. $Na<Mg>Al<Si$
• C. $Na>Mg>Al>Si$
• D. $Na>Mg>Al<Si$

Answer 1

The correct option is B $Na<Mg>Al<Si$

All elements in the given question belong to the same period so, as we move across the period nuclear charge increases, atomic size decreases hence ionization energy generally increases.

$Na-\left[Ne\right]3s^1$

The electron must be removed from partially filled $‘3s^{\prime }$ orbital which would result in a stable noble gas electronic configuration.

$Al-\left[Ne\right]3s^{2}3p^1$

The electron has to be removed from partially filled ${}^{\prime }3p^{\prime }$ orbital which would result in a stable electronic configuration.

$Mg-\left[Ne\right]3s^2$

The electron has to be removed from fully filled, stable $‘3s^{\prime }$ orbital. which will result in unstable electronic configuration. so, ionisation energy of $Mg>Al$.

$Si-\left[Ne\right]3s^{2}3p^2$

The electron has to be removed from partially filled ${}^{\prime }3p^{\prime }$ orbital.

Removal of an electron from a stable fully filled orbital requires more energy than a partially filled or half-filled orbital. So, correct order of $1St$ ionization enthalpy is, $Na<Mg>Al<Si$

Hence, option (A) is correct.