Question
Which of the elements $Na , Mg , Si$ and P would have the greatest difference between the first and second ionisation enthalpies. Briefly explain your answer.
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Answer
Among the given elements Na being alkali metal has only one electron in the valence shell, therefore has very low $\Delta_{ i } H _1$.
However, after the removal of one electron, it acquires nearest inert gas or neon gas configuration, i.e. $Na +\left(1 s^2, 2 s^2, 2 p ^2\right)$. Therefore, its $\Delta_{ i } H _2$ is expected to be very high. Consequently, the difference in first and second ionisation enthalpies would be greatest in case of Na.
However, it may be noted here that in case of $Mg , Si$ and P , although their $\Delta_{ i } H _1$ will be much higher than that of Na but their $\Delta_{ i } H _2$ be much lower than that of Na . As a result, the difference in their respectively, $\Delta_{ i } H _1$ and $\Delta_{ i } H _2$ would be much lower than that of Na .
However, after the removal of one electron, it acquires nearest inert gas or neon gas configuration, i.e. $Na +\left(1 s^2, 2 s^2, 2 p ^2\right)$. Therefore, its $\Delta_{ i } H _2$ is expected to be very high. Consequently, the difference in first and second ionisation enthalpies would be greatest in case of Na.
However, it may be noted here that in case of $Mg , Si$ and P , although their $\Delta_{ i } H _1$ will be much higher than that of Na but their $\Delta_{ i } H _2$ be much lower than that of Na . As a result, the difference in their respectively, $\Delta_{ i } H _1$ and $\Delta_{ i } H _2$ would be much lower than that of Na .
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