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The ionisation energy of a hydrogen atom is 13.6 eV. What is the energy of the ground state of doubly ionised lithium (Li²+)?

1. -28.7 eV
2. -54.4 eV
3. -122.4 eV
4. -13.6 eV

Correct answer: -122.4 eV

Solution

For a hydrogen-like ion with atomic number Z, the ground state energy is E₁ = -13.6 * Z² eV. For Li²+ (Z=3): E₁ = -13.6 * 9 = -122.4 eV.

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