Question 5
These four elements are all in period 3. Which has the highest first ionization energy?
The right answer is the one marked above.
Argon, at 1521 kJ mol−1 — the largest figure in the row, and by a wide margin.
Argon is the group VIII element of period 3, and every period peaks at its group VIII element. Notice too that the rise from left to right is not perfectly smooth — aluminium (577) is lower than magnesium (738) — but the overall pattern across the period holds.