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The ionisation energy of hydrogen atom is 13.6 eV, the ionisation energy of helium atom would be

1. 13.6 eV
2. 27.2 eV
3. 6.8 eV
4. 54.4 eV

Correct answer: 54.4 eV

Solution

The ionisation energy of a hydrogen-like atom is proportional to the square of its atomic number (Z). For helium (Z=2), the ionisation energy is 13.6 × Z² = 13.6 × 4 = 54.4 eV.

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