Question

how to compare first IE of Na, Mg, Al ,Si

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

Answer 1

Answer: (C)

Na < Al < Mg < Si

Answer 2

The first ionization energy (IE) of elements generally increases across a period due to increasing nuclear charge. Na, Mg, Al, and Si are in the 3rd period. Their electronic configurations are: Na: [Ne] 3s^1 Mg: [Ne] 3s^2 Al: [Ne] 3s^2 3p^1 Si: [Ne] 3s^2 3p^2

Na has the lowest IE due to its single valence electron. While IE increases across the period, there's an exception between Mg and Al. Mg has a stable, filled $3s^2$ subshell, requiring more energy to ionize than Al, which has a $3p^1$ electron in a higher energy orbital. Thus, IE(Al) < IE(Mg). Si, with a higher nuclear charge and electron in the $3p$ subshell, follows the trend, having a higher IE than Al. Therefore, the order is Na < Al < Mg < Si.