Field of Chemistry is all about of the study of composition of existing substances and synthesis of newer substances. Aufbau’s principle predicts Valency and electron configuration that are responsible factors for formation of newer substances. In this article, I’m going to provide a detailed explanation of Aufbau’s principle.
Aufbau’s principle highlights the proper selection of atomic orbits in electron configuration. So, to understand the mystery of filling atomic orbitals by electrons it becomes inevitable of explanation of Aufbau’s principle in detail.
Many prominent scientists like Neil Bohr, Wolfgang Pauli, Friedrich Hund, Lewis have presented different models and theories for proper electron configuration.
Among those prominent works, the Aufbau’s principle formulated by Neil Bohr, was a major breakthrough for determining proper sequence of atomic orbitals that are occupied by electrons.
What is Aufbau’s principle :
Aufbau is not a name of scientist but it is a German word made from two root words in German language.
- Auf : up
- bau : build or construct
- Aufbau : Means building up something.
This principle was formulated by Great Danish physicist Neil Bohr about 1920 while applying laws of quantum mechanics.
Statement of Aufbau’s principle
Aufbau’s principle states that “each electron occupies lowest energy orbital available.” Lowest energy orbit is ground state energy orbit.
So it becomes prerequisite to determine the correct sequence of atomic orbitals from lowest energy to highest energy. This sequence is shown by a diagram know as Aufbau’s diagram.
Aufbau’s atomic orbital sequence diagram.
1s^2 2s^2 2p^6 3s^2 3p^6 4s^2 3d^10 4p^6. 5s^2 4d^10 5p^6 6s^2 4f^14 5d^10 6p^6. 7s^2 5f^14 6d^10 7p^6 8s^2 ………. ………. and goes on.
This shows an arrangement of increasing energy orbit system from 1s to 7s orbitals and so on…. It governed by “Medelung rule”
What is Medlung Rule?
Medlung rule provides hierarchy of energy orbital 9n the basis of sum of (n+l) where, n = principle quantum number and l = azimuthal quantum numbers as 0, 1, 2, 3 correspond to s, p, d and f orbitals respectively. Sum of (n+l) determine the level of energy between two orbitals.
Lower the sum lower will be the energy level of orbits to enter electrons.
Examples of Medlung rule
For example, in 3d and 4s orbitals,3d= (n+l) = 3 +2 = 5 4s= (n+l) = 4 +0 = 4 So, sum of (n+l) for 4s orbital is lower than 3d orbital. Hence electrons enter into 4s before 3d orbital.
It is important to note that when sum is equal for both orbitals then electron prefers to enter in lower orbit number.
For example, for 3d and 4p orbitals
- For 3d orbital, (n+l) = 3 + 2 = 5
- For 4p orbital, (n+l) = 4 + 1 = 5
Here, sum of quantum numbers is equal for both of the orbitals. At this place, electron prefer to enter in 3d orbital as lower principle quantum number (n =3).
Importance of Aufbau’s principle
- This principle facilities to place electrons in various sub-shells or orbitals in case of multi-electron elements.
- It helps to predict type and nature of chemical bond that an atom is going to form.
- It attempts to determine possible valancy of an atom.
Limitations of Aufbau’s Principle
Aufbau’s Principle is not a law and its predictions are not always correct. It works very well for ground state electron orbits for first 18 elements in periodic table. For following 100 elements it doesn’t work very well.
There are some exceptions to this principle. Transition metals, lanthenides and actinides do not obey Aufbau’s principle and show different electron configuration other than predicted by Aufbau.
Exception of Chromium
Chromium is a transition metal having atomic number 24. According to Aufbau’s principle it’s electron configuration should be
1s^2 2s^2 2p^6 3s^2 3p^6 4s^2 3d^4
But its actual electron configuration is as follows
1s^2 2s^2 2p^6 3s^2 3p^6 4s^1 3d^5
It means that before fulfilling 4s orbital completely electrons complete partial fulfilment of 3d orbital. It is because half filled 3d orbital is more stable than partial fulfilment of 4s orbital.
Exception of Cooper
Second example is of Copper. Copper having 29 electrons must show following electron configuration according to Aufbau’s principle
1s^2 2s^2 2p^6 3s^2 3p^6 4s^2 3d^9
Instead, Copper shows different electron configuration as follows
1s^2 2s^2 2p^6 3s^2 3p^6 4s^1 3d^10
It mean electrons before fulfilling lower ground state orbital 4s they fulfil higher energy orbital 3d completely. Which is totally against what is said by Aufbau’s principle.
Here, fulfilling 3d orbital completely provides much more stability for Copper atom than the 4s orbital.
It was all about the explanation of Aufbau’s principle and its limitations. Hope, it will help you.
Summary of explanation of Aufbau’s principle
Aufbau’s principle predicts Valency and electron configuration that are responsible factors for formation of newer substances.
It highlights the proper selection of atomic orbits in electron configuration.
Neil Bohr, Wolfgang Pauli, Friedrich Hund, Lewis have presented different models and theories for proper electron configuration.
Aufbau’s principle is formulated by Neil Bohr. This principle was formulated by Great Danish physicist Neil Bohr about 1920 while applying laws of quantum mechanics.
Statement of Aufbaus principle is that “each electron occupies lowest energy orbital available.”
It helps to predict type and nature of chemical bond that an atom is going to form.
There are some exceptions to this principle. Transition metals, lanthenides and actinides do not obey Aufbau’s principle and show different electron configuration other than predicted by Aufbau.
Explanation of Dalton’s atomic theory