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Ionisation energy of He⁺ is 19.6 × 10^−18 J atom^−1. The energy of the first stationary state (n = 1) of Li²+ is

1. 4.41 × 10^−16 J atom^−1
2. −4.41 × 10^−17 J atom^−1
3. −2.2 × 10^−15 J atom^−1
4. 8.82 × 10^−17 J atom^−1

Correct answer: −4.41 × 10^−17 J atom^−1

Solution

For one-electron ions E(n=1) is proportional to Z^2 and is negative. E(Li2+) = -(19.6e-18)*(3^2/2^2) = -19.6e-18 * 9/4 = -4.41e-17 J/atom.

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