Question 2
Sodium has atomic number 11. What is its electron configuration?
The right answer is the one marked above.
2, 8, 1. Eleven electrons, filled from the inside: 2 fills the first level, 8 fills the second, and the last one starts the third.
The order of the numbers is not free — they are always written from the nucleus outwards, so 1, 8, 2 says something impossible: it puts 8 electrons in a level that holds at most 2. And 2, 9 breaks the second level's maximum of 8.