Practice question · Numerical answer
Successive ionisation energies of an element (kJ/mol) are 738, 1451, 7733, 10540. Which group number does the huge jump identify, that is, how many valence electrons does the atom have?
Hints
- Find where the cost suddenly multiplies rather than merely rising.
- The jump from 1451 to 7733 comes after two electrons have been removed.
Show the answer
2
Why
The first two electrons cost 738 and 1451 kJ/mol; the third costs over five times as much because it breaks into the noble-gas core. Two valence electrons means group 2, this is magnesium.
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