The exclusion principle and the periodic table
Exclusion principle: No two electrons can have the same state (no two electrons can have the same set of quantum numbers) (Wolfgang Pauli, 1924)
atomic shells: a group of allowed states, where the energy is very close
chemical properties determined by the number of electrons in the outermost incomplete shell
first shell: possible states: n=1, l=0, ml=0, ms=+1/2 or -1/2 (total: 2)
second shell: n=2, l=0, ml=0, ms=+1/2 or -1/2
n=2, l=1, ml=1,0,-1, ms=+1/2 or -1/2 (total: 8)
third shell: n=3, l=0, ml=0, ms=+1/2 or -1/2
n=3, l=1, ml=1,0,-1, ms=+1/2 or -1/2 (total: 8)
fourth shell: n=4, l=0, ml=0, ms=+1/2 or -1/2
n=3, l=2, ml=2,1,0,-1,-2, ms=+1/2 or -1/2
n=4, l=1, ml=1,0,-1, ms=+1/2 or -1/2 (total: 18)
filling of the states with electrons, after increasing energy of the states (electrons always occupy the most lower allowed energy state)