how many electrons are there in the 3rd principal energy level (n=3) of a phosphorus atom?
This is known as the Pauli exclusion principle. Bohr model predicts that a maximum of 2(3)2 = 18 electrons can reside in the 3rd principal energy level. Quantum Mechanical Model of the atom predicts that in principal energy level 3, there are 3s, 3p, and 3d. In any s-subshell, there is 1 atomic orbital.