JEE Main 2022 · 27 June · morning · Single correct · +4 / −1
Given below are two statements : one is labelled as Assertion (A) and the other is labelled as Reason (R).
Assertion (A) : The ionic radii of O²⁻ and Mg²⁺ are same.
Reason (R) : Both O²⁻ and Mg²⁺ are isoelectronic species.
In the light of the above statements, choose the correct answer from the options given below.
- (a) Both (A) and (R) are true and (R) is the correct explanation of (A).
- (b) Both (A) and (R) are true but (R) is not the correct explanation of (A).
- (c) (A) is true but (R) is false.
- (d) (A) is false but (R) is true.
Answer and solution
Answer (d) (A) is false but (R) is true.
Solution
Chemistry ma’am · 4 steps
Same number of electrons does not mean same size. Count the protons pulling on them.
- Check the Reason. O has 8 electrons, so O²⁻ has 8 + 2 = 10. Mg has 12, so Mg²⁺ has 12 - 2 = 10. Both have 10 electrons, so they are isoelectronic and (R) is true.
- What sets the size. In isoelectronic species the electron cloud is the same, so the nuclear charge decides the size. More protons pull the same 10 electrons in tighter, which gives a smaller ion.
Ion Protons (Z) Electrons Ionic radius O²⁻ 8 10 about 140 pm Mg²⁺ 12 10 about 72 pm - Check the Assertion. Mg²⁺ has 12 protons and O²⁻ has only 8, so Mg²⁺ is much smaller. The radii are not the same, so (A) is false.
- So. (A) is false and (R) is true, which is option (d).
The idea to keep. In an isoelectronic series, the radius falls as the nuclear charge rises. The ion with more protons is always the smaller one.
Answer from the NTA final answer key. Solution by PrepWiser.