Answer :
Rule 1. aufbau rule - energy levels filled so lowest energy level filled first.
Rule 2. pauli exclusion principle - maximum of 2 electrons may be placed in each energy level. Provided the spins of electrons are of opposite sign.
Rule 3. hunds rule - when energy levels are equal one electron is added to each energy level in turn with their spins unpaired until each energy level has one electron.
Rule 2. pauli exclusion principle - maximum of 2 electrons may be placed in each energy level. Provided the spins of electrons are of opposite sign.
Rule 3. hunds rule - when energy levels are equal one electron is added to each energy level in turn with their spins unpaired until each energy level has one electron.