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The first four successive ionisation energies of an element are 191, 578, 872, and 5962 kcal/mol. How many valence electrons does this element have?

1. 1
2. 2
3. 3
4. 4

Correct answer: 3

Solution

The sharp increase between the 3rd and 4th ionisation energies (872 to 5962 kcal/mol) indicates that the 4th electron is being removed from an inner (core) shell, which requires much more energy. Therefore, the element has 3 valence electrons.

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