MCQ
The first ionisation potential of $Na, Mg, Al$ and $Si$ are in the order :-
- ✓$Na < Mg > Al < Si$
- B$Na > Mg > Al > Si$
- C$Na < Mg < Al < Si$
- D$Na > Mg > Al < Si$
✓
Answer
Correct option: A.
$Na < Mg > Al < Si$
a
As we move across the period, nuclear charge increases, atomic size decreases hence, ionization enthalpy increases.
As we move across the period, nuclear charge increases, atomic size decreases hence, ionization enthalpy increases.
For $A l\left(3 s^{2} 3 p^{1}\right)$, electron has to be removed from partially filled $3 p$ orbital whereas in $M g\left(3 s^{2}\right)$, electron has to be removed from stable fully filled $3 s$ orbital.
Removal of an electron from stable, fully filled orbital requires more energy than removal of electron from partially filled orbital.
Thus, ionisation enthalpy for $M g$ is greater than ionisation enthalpy for $A l$.
So, the correct order of first ionization enthalpies is: $N a
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