rₙ = 52.9 n²/Z pm. For He⁺: r₂ = 52.9 × 4/2 = 105.8 pm ✓. For Li²⁺ (Z=3), n=3: r₃ = 52.9 × 9/3 = 52.9 × 3 = 158.7 pm
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