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Consider the elements Mg, Al, Si, P and Si, the correct increasing order of their first ionization enthalpy is:

1. Mg < Al < Si < S < P
2. Al < Mg < Si < S < P
3. Mg < Al < Si < P < S
4. Al < Mg < S < Si < P

Correct answer: Al < Mg < Si < S < P

Solution

Across period 3, IE rises but with two dips: Al < Mg because Al loses a 3p electron (higher energy, easier) vs Mg's filled 3s; and S < P because P has a stable half-filled 3p^3. So the order is Al < Mg < Si < S < P.

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