Question

Which of the following species will have the smallest size ?
A. `Li^(+)`
B. `Mg^(2+)`
C. `Al^(3+)`
D. `K^(+)`

Answer 1

Correct Answer - 3
`Al^(3+)`
`K^(+)` has more number of shells than `Mg^(2+)` and `Al^(3+),Al^(3+)` and `Mg^(2+)` are isoelectronic but `Al^(2+)` has higher nuclear charge so `Al^(3+) lt Mg^(2+), Mg^(2+)` and `Li^(+)` has diagonal relationship. But due to `+2` charge in `Mg^(2+),` the `Mg^(2+)` is smaller than `Li^(+)` .Hence `Al^(3+)` is the smallest one.
`K^(+)=1.38Å, Li^(2+)=0.76Å, Mg^(2+)=0.72Å` and `Al^(3+)=0.535Å`.