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The difference between the radii of 3rd and 4th orbits of Li2+ is ΔR1. The difference between the radii of 3rd and 4th orbits of He+ is ΔR2. ΔR1: ΔR2 is:

1. 8: 3
2. 2: 2
3. 3: 8
4. 2: 3

Correct answer: 2: 3

Solution

Radius r = 0.529 n^2/Z. ΔR(3->4) = (16-9)/Z = 7/Z. For Li2+ (Z=3): ΔR1 = 7/3; for He+ (Z=2): ΔR2 = 7/2. ΔR1:ΔR2 = (7/3):(7/2) = 2:3.

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