Question 6
Which of these electron configurations is impossible?
The right answer is the one marked above.
2, 9, 1. The second level holds at most 8 electrons, so 9 cannot be there.
The other three are all real: 2, 8, 4 is silicon; 2, 8, 8, 2 is calcium; 2, 5 is nitrogen. Test any configuration by checking each level against its maximum — 2, 8, 18, 32 — and remembering the last level may hold no more than 8.