The Aufbau principle is an important principle in Chemistry that helps explain the electronic structure of atoms and molecules. It is also sometimes referred to as the Aufbau rule or the building up principle. This principle states that “Electrons orbiting one or more atoms fill the lowest energy level before filling higher level”. Thus in a molecule, the atoms, electrons and ions attain the most stable electronic configuration that is possible.

Did you know: Aufbau is a German word that means ‘building up’.

The Aufbau principle is closely related to other principles such as Pauli’s exclusion principle and Hund’s rule that also talk about how electrons fill up the orbits around the nucleus in an atom.

Aufbau principle diagram
An electronic configuration can be represented using the Aufbau diagram. The order in which the orbitals are filled is given by Madelung rule also sometimes referred to as the diagonal rule. The rule is based on the total number of nodes in an atomic orbital.

According to Madelung’s rule, If n is the principal quantum number and l is the azimuthal quantum number, the energy of an atomic orbital increases with increasing n+l. In case n+l have identical values then the energy increases with increasing n.

The lower orbital are filled before the higher orbitals are filled.

Aufbau Principle | Image source: wikipedia.org

In order to draw the aufbau diagram follow the below steps:

  • First determine the number of electrons in the atom
  • The orbitals are filled in the 1s,2s,2p,3s,3p,4s,3d,4p,4s order.
  • Fill the first two electrons in the orbital in first energy level (1s)
  • Now fill the s orbital in the second energy level (2s) with two electrons.
  • Put one electron in each of the three p orbitals in the second energy level (2p). If there are any remaining electrons place a second electron in each of the 2p orbitals to complete the electron pairs.
  • Continue in this way through each of the successive energy levels until all the electrons are filled

Aufbau principle examples:
Refer to the following Aufbau principle example to better understand the concept.

Example: Electronic configuration for Silicon using Aufbau principle

  • Silicon has 14 electrons. The lowest energy level is filled first.
  • As per the rules given above, first fill the 1s level with two electrons leaving 12 electrons to be filled in the other levels. This is shown in Step A.
  • Next the 2s orbital is filled with two electrons. There are 10 electrons to be filled now as shown in Step B.
  • The 2p orbital is the next energy level can hold upto 6 electrons as depicted in Step C. This leaves 4 electrons to be filled.
  • As shown in Step D now fill the next energy level 3s with two electrons.
  • Now there are only 2 electrons that have to be placed. The next energy level is 3p. The orbitals are filled with one type of spin before the other spin appears. In this case two spin electrons are placed in the 3p orbital as shown in Step E.

 
Silicon Electronic Configuration | Image source: thoughtco.com

The electron are placed as follows 1s,2s,2p,3s,3p. The electronic configuration for silicon is 1s22s22p63s23p2.

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