The atomic number of fluorine is 9. How many electrons are in the second principal energy level in the ground state?

• A. 2
• B. 7
• C. 9
• D. 4

Answer: (B)

Electrons fill the lowest available energy levels first, and the second principal energy level contains the 2s and 2p subshells (up to 2 in 2s and up to 6 in 2p, total 8). Fluorine has 9 total electrons. The first energy level holds 2 electrons in 1s, leaving 7 to place in higher levels. Those 7 occupy 2s with 2 electrons and 2p with 5 electrons, giving 2 + 5 = 7 electrons in the second energy level. So there are seven electrons in the second level.