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. $Na<Mg<Al<Si$
• D. $Na>Mg>Al<Si$

Answer 1

Answer: (A)

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