Afbau principle

                                               Afbau Principle

    The word afbau has been derived from German word and has the meaning building or construction.It states that ,"the electron are filled in an atomic orbital in the order of their increasing energy." According to their principle the electron first enter in the subshell with minimum energy and them in the sub shell with next higher energy.

               The principle takes its name from the German, Aufbauprinzip, "building-up principle", rather than being named for a scientist. In fact, it was formulated by Neils Bohrs  and Wolfgang Pauli  in the early 1920s, and states that:

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The orbitals of lower energy are filled in first with the electrons and only then the orbitals of high energy are filled.

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This was an early application of  quantum mechanics to the properties of electron, and explained chemical properties in physical terms. Each added electron is subject to the electric field created by the positive charge of the atomic nucleus and the negative charge of other electrons that are bound to the nucleus. Although in hydrogen there is no energy difference between orbitals with the same principal quantum number n, this is not true for the outer electrons of other atoms.

In the old quantum theory prior to quantum mechanics, electrons were supposed to occupy classical elliptical orbits. The orbits with the highest angular momentum are 'circular orbits' outside the inner electrons, but orbits with low angular momentum (s- and p-orbitals) have high orbital eccentricity, so that they get closer to the nucleus and feel on average a less strongly screened nuclear charge.

                          The relative energy of an orbital is determine by the principle and azimuthal quantum number and popularly known as n+l rule. It has two parts;

1.     The higher value of n+l have higher energy level and lower value have lower energy level. For example;                                                                                       Subshell                                     n+l                                                                                 1s                                           1+0=1                                                                              2s                                           2+0=2                                                                              2p                                           2+1=3                                                                            3s                                           3+0=3                                                                              3p                                           3+1=4                                                                            3d                                           3+2=5                                                                            4s                                           4+0=4                                                                              4p                                           4+1=5                                                                            4d                                           4+2=6                                                                            4f                                            4+3=7                                                                                                                          and so on.                                                                                   Hence,the order of energy level in different orbitals is, 1s<2s<2p<3s<3p<4s<3d<4p<5s<4d and so on. Therefore, the 4s orbital filled before 3d.                      

2.     In case of same value of (n+l), the lower value of n has the lower energy level and higher ones have higher energy level.                                                                            For ex; 2p (2+1=3) orbitals are filled before 3s (3+0=3)                                                          3p(3+1=4) orbitals are filled before 4s (4+0=4)                                                            3d (3+2=5) orbitals are filled before 4p (4+1=5)                                                                                                                                                                                                                            Hence, the lower energy level are better place for the electrons and occupied first.The relative energy of different orbitals can be shown below;                                                                               

1s

           2s           2p

                          3s          3p       3d

                                         4s        4p          4d           4f

                                                                     5s            5p             5d            5f

                                                                                      6s              6p           6d            6f

                                                                                                                                                                                                                                                                                                          and so on.  

                    

Reference                                                                                      

Wikipidea                                                                                     

Dasu Paudel