Aufbau Rule

                             The molecular orbitals are filled in the increasing order of their energies starting with orbital of least energy. This principle is used to determine the electronic configuration of an atom, molecule, or ion. In German Aufbau meaning "building up, construction " . This is other wise called as aufbau rule Or building- up principle.
                            These orbitals are filled in the order of n+l rule. n+l rule also known as Madelung rule or klechkowski rule. Orbitals with lower n+l value are filled before those with higher n+l value.

                   n   ===  Principle quantum number.
                   l    ===  Azimuthal quantum number.
the value l=0,1,2,3. corresponds to the s,p,d and f labels respectively.

Example :
2p and 3s orbitals.
 In 2p orbital n=2 and l=2 .
In 3s orbital n=3 and l=1.
( n+l=4 ) in both cases.
But the 2p orbital has the lower energy or lower n value and will get filled before the 3s shell.
                       
                                      The order of the orbital energies from lowest to highest.

  no^{e  ===  electronic configuration is represented .}

^{n ---------- energy level}

^{o ---------- orbital type ( s, p, d and f )}

^{e ---------- no. of electrons in that orbital shell.}

^{Example :}

^{Oxygen has 8 protons and 8 electrons. First 2 electrons would fill the 1s orbitals. Next 2 would fill the 2s orbitals and the remaining 4 electrons to take spots in the 2p orbitals.}

^{Electronic configuration of oxygen is}  1s² 2s² 2p⁴