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The word Aufbau means build up. According to this principle the first electron in an atom occupies the quantum state with lowest possible energy and then the second electron goes to the next quantum state having higher energy. The model of the many electron atom which Niels Bohr constructed in order to explain the structure of the periodic table was based on detailed spectroscopic and chemical evidence.

The principle behind the model is the Aufbau principle which implies the considerations in the terminology based on orbitals of the central field form rather than the classical orbits used by Bohr.

Aufbau Principle Definition

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What is Aufbau Principle?

"The lowest orbits in atoms will fill with electrons before higher orbits do."

Aufbau is a German word which means building up or construction. This is the reason that this principle is also known as building up principle or the construction principle. To define Aufbau principle this principle gives us a sequence in which various orbitals are filled up with the electrons in the increasing order of their energy. The relative order of energy of various orbitals of an atom is given as

Aufbau Principle

According to the Aufbau Principle the sequence in which various orbitals are filled up with electrons are represented above.

Exceptions to the Aufbau Principle

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There are number of exceptions to the electronic configurations predicted from Aufbau principle. For d and f block elements orbital interactions cause exceptions to the Aufbau principle. Half-filled and completely filled d and f sub shells have extra stability.

As we proceed to fill the 3d set of orbitals from Sc (Z=21) to Zn (Z=30) we see that these orbitals are not filled regularly. Some sets of orbitals are quite close in energy and the energy difference between them varies with atomic number.

Energy Difference

For example, Cr the Aufbau prediction for the valence configuration is 4s23d4, while that observed is 4s13d5. The diagrams are given below.

Aufbau Prediction


The electron electron repulsion is reduced in the observed configuration, since each electron has its own seat.

Aufbau Rule

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The filling of different sub shells by electrons are according to increasing order of energy and follows (n+1) rule.

The (n+1) rule states
  1. Electrons are filled in the increasing order of (n+1) value of each sub shell.
  2. In case two sub shells have same value of (n+1) then the sub shell with lower value of n, out of these is considered of lower energy and filled prior to the other.


(n+1) value 1
2
3
4
5
6
7
8
1s
2s
2p

3s 3p3d

4s
4p
4d4f

5s 5p 5d5f

6s6p 6d
7s
7p
8s


Hence increasing order of energies of different sub shells are

1s<2s<2p<3s<3p<4s<3d<4p<5s<4d<5p<6s<4f<5d<6p<7s<5f<6d<7p<8s

Aufbau Principle Diagram

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According to Aufbau principle the orbitals are filled up in the order of increasing energy that is the orbital with lowest energy is filled up first following the higher energy orbital.

The order in which the orbitals are filled with electrons. This order can also be represented by electronic configuration diagram starting from the top the direction of the arrows gives the order of filling of orbitals. The Aufbau principle chart is given below.

Aufbau Principle Diagram

This order shows that the energies of the orbital of the same main energy level increases as the value of l increases. The total energy of an atom however depends not only on the sum of the orbital energies but also on the energy of repulsion between the electrons in these orbitals.

Aufbau Principle Examples

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The Aufbau principle example are given below.

Solved Examples

Question 1: Using orbital energy diagrams draw the electron configuration of Fluorine and neon.
Solution:
 
The orbital energy diagram is as follows.

Second Period Elements
 

Question 2: Draw the relative energies of s, p, and d orbitals in multi electron atoms.
Solution:
 
The energy diagram is an follows.

Multi-electron Atoms