Question: how many electrons are needed to fill the third main energy level?
Answer: The five d orbitals can hold up to 10 electrons. Thus, the third level holds a maximum of 18 electrons: 2 in the s orbital, 6 in the three p orbitals, and 10 in the five d orbitals. The fourth and higher levels also have an f sublevel, containing seven f orbitals, which can hold a maximum of 14 electrons.