Aufbau Principle

H He
Li Be B C N O F Ne
Na Mg Al Si P S Cl Ar
K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn       

The Aufbau (building up) Principle:

  1. the number of electrons in an atom is equal to the atomic number;
  2. each added electron will enter the orbitals in the order of increasing energy;
  3. an orbital cannot take more than 2 electrons.