Electron shell of heavy atoms

Two basic rules: The number of electrons in subshell is 2(2l+1). Atomic shells ale occupied as follows: 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, … (the 3d state has higher energy than the 4s due to the higher value of I, so it gets occupied later). The Pauli exclusion principle and Hund's rule apply here (= as long as possible, electrons in an atom remain unpaired, i.e. have parallel spins. Such electrons are more separated from each other due to the same magnetic quantum number => this state has lower energy).
 * 1) a system of particles is stable if its total energy is minimal (is this is not the case, the system tries to reach a state with the lowest possible energy),
 * 2) in each quantum state (determined by 4 parameters), only one electron may exist in one system (electron shell)(see Quantum numbers).