JEE Main 2021 · 25 July · evening · Single correct · +4 / −1
The ionic radii of F⁻ and O²⁻ respectively are 1.33Aᵒ and 1.4Aᵒ, while the covalent radius of N is 0.74Aᵒ.
The correct statement for the ionic radius of N³⁻ from the following is :
- (a) It is smaller than F⁻ and N
- (b) It is bigger than O2⁻ and F⁻
- (c) It is bigger than F⁻ and N, but smaller than of O²⁻
- (d) It is smaller than O²⁻ and F⁻, but bigger than of N
Answer and solution
Answer (b) It is bigger than O2⁻ and F⁻
Solution
Chemistry ma’am · 3 steps
N³⁻, O²⁻ and F⁻ all have 10 electrons, so whichever ion has the fewest protons is the biggest.
- Same electron count. N³⁻ has 7 + 3 = 10 electrons, O²⁻ has 8 + 2 = 10 and F⁻ has 9 + 1 = 10. They are isoelectronic, so only the nuclear charge decides their size.
Ion Protons Electrons Pull per electron Ionic radius N³⁻ 7 10 weakest largest (about 1.71 Å) O²⁻ 8 10 medium 1.40 Å F⁻ 9 10 strongest 1.33 Å - Compare with N atom. When N gains 3 electrons, its 7 protons have to hold 10 electrons instead of 7. Electron repulsion increases and the cloud spreads out, so N³⁻ is much bigger than the N atom (0.74 Å).
- So. N³⁻ is the largest of the three ions. It is bigger than both O²⁻ and F⁻, which is option (b).
The idea to keep. In an isoelectronic series, fewer protons means a bigger ion. An anion is always larger than its parent atom.
Answer from the NTA final answer key. Solution by PrepWiser.