JEE Main levelSingle correct optionDifficulty 2 of 3pt016
The ion MX3+ is isoelectronic with argon. The element M lies in
Agroup 3, period 4
Bgroup 13, period 3
Cgroup 15, period 3
Dgroup 3, period 3
Answer: A
Ar has 18 electrons, so M has 18+3=21 electrons (Z = 21, scandium) with configuration [Ar]3d14s2. This is group 3, period 4. AlX3+ is isoelectronic with neon, not argon.