Question

The first ionization enthalpies of $Na,Mg,Al$ and $Si$ are in the order:

• A. $Na<Mg>Al<Si$
• B. $Na>Mg>Al>Si$
• C. $Si>Al>Mg>Na$
• D. $Na>Al>Si>Mg$

Answer 1

The correct option is A $Na<Mg>Al<Si$

As we move across the period, nuclear charge increases, atomic size decreases hence, ionization enthalpy increases. For $Al$ ($3s^{2}3p^1$ ), electron has to be removed from partially filled $3p$ orbital whereas in $Mg(3s^2$), electron has to be removed from stable fully filled $3s$ 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 $Mg$ is greater than ionisation enthalpy for $Al$. So, the correct order of first ionization enthalpies is: $Na<Mg>Al<Si$