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(B) The first ionisation energy generally increases across a period due to an increase in effective nuclear charge. Exceptions occur when a more stabl

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$Na$ and $Mg$

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(B) The first ionisation energy generally increases across a period due to an increase in effective nuclear charge. Exceptions occur when a more stable electronic configuration (like fully filled or half-filled orbitals) is present in the preceding element. $Be$ $(2s^2)$ has a higher ionisation energy than $B$ $(2s^2 2p^1)$. $Mg$ $(3s^2)$ has a higher ionisation energy than $Al$ $(3s^2 3p^1)$. $N$ $(2s^2 2p^3)$ has a higher ionisation energy than $O$ $(2s^2 2p^4)$. $Na$ $(3s^1)$ has a lower ionisation energy than $Mg$ $(3s^2)$,which follows the general trend. Therefore,$Na$ and $Mg$ is not an exception.

Generally,the first ionisation energy increases along a period. But there are some exceptions. The one which is not an exception is

Similar Questions

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The first ionization potential $(IP)$ of $Be$ $(Z=4)$ is higher than which of the following elements?
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$(e) K (Z=19)$

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