Question 4
Chlorine has atomic number 17. What is its electron configuration?
The right answer is the one marked above.
2, 8, 7. Seventeen electrons: 2 in the first level, 8 in the second, and the remaining 7 in the third.
Check the total every time — 2 + 8 + 7 = 17, which must equal the atomic number. That single check catches most mistakes. 2, 8, 6 is sulphur (16) and 2, 8, 8, 1 is potassium (19).