According to Neils Bohr, the atomic structure is described as electrons orbiting around the nucleus in orbitals. Each orbital holds two electrons that are oppositely spinning, according to Pauli's Exclusion Principle. These orbitals are then located in sub-shells of varying energy levels. There are 4 types of these sub-shells. In order of decreasing energy, they are the s, p ,d and f sub-shells. They differ in shape and energy levels. The s sub-shell holds 1 orbital, p holds 3, d holds 5 and f holds 7 orbitals. These sub-shells are oriented according to their principal quantum number, n. There are a total of 7 energy levels with 1 being the strongest and closest to the nucleus, and 14 being the weakest and farthest to the nucleus. You will see that electrons are filled from the strongest 1s orbitals, to the weakest 7f orbital.
Thus, in the energy level n=3, there are 3s, 3p, 3d and 3f sub-shells. The total electrons for this energy level is. therefore,
Total electrons = 1(2) + 3(2) + 5(2) + 7(2) = 32 electrons
The 2rd energy level can hold up to 16 orbitals containing two electron each. That would be 32 electrons all in all.
