The first ionization enthalpy $(\Delta_{i} H)$ values of the third period elements,$Na, Mg$ and $Si$ are respectively $496, 737$ and $786 \, kJ \, mol^{-1}$. Predict whether the first $\Delta_{i} H$ value for $Al$ will be more close to $575$ or $760 \, kJ \, mol^{-1}$? Justify your answer.
(A) The first ionization enthalpy for $Al$ will be closer to $575 \, kJ \, mol^{-1}$.
In the third period,the ionization enthalpy generally increases from left to right due to an increase in effective nuclear charge.
However,$Mg$ $(3s^2)$ has a fully filled orbital,making it more stable than $Al$ $(3s^2 3p^1)$.
Therefore,the first ionization enthalpy of $Al$ is lower than that of $Mg$ $(737 \, kJ \, mol^{-1})$.
Since $575 \, kJ \, mol^{-1}$ is less than $737 \, kJ \, mol^{-1}$,it is the correct prediction.
In the third period,the ionization enthalpy generally increases from left to right due to an increase in effective nuclear charge.
However,$Mg$ $(3s^2)$ has a fully filled orbital,making it more stable than $Al$ $(3s^2 3p^1)$.
Therefore,the first ionization enthalpy of $Al$ is lower than that of $Mg$ $(737 \, kJ \, mol^{-1})$.
Since $575 \, kJ \, mol^{-1}$ is less than $737 \, kJ \, mol^{-1}$,it is the correct prediction.
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