The first ionisation enthalpy of magnesium is higher than that of sodium. On the other hand, the second ionisation enthalpy of sodium is very much higher than that of magnesium. Explain.

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Solution

$I E_{1}$ of $M g > I E_{1}$ of $N a$
because the outermost orbital of $M g$ is fully filled, thus have high first ionisation enthalpy.
$I E_{2}$ of $M g < I E_{2}$ of $N a$
because after removal of an $e^{-}$ $N a$ , has completely filled outermost electronic configuration from which removing an $e^{-}$ becomes tough. Hence $I E_{2}$ of $N a$ is higher than $M g$

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