Question

Which represents the correct order of first ionisation potential of third period elements?

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

Answer 1

Answer: (D)

Generally, ionisation potential increases, as we move from left to right in a period. However, the ionisation potential of II group elements is higher than that of the III group elements. This is because of the fully filled $s$-orbitals in II group elements.
Hence, the correct order of ionisation potential is
$Na<Al<Mg<Si$