# Aufbau Principle - Definition, Diagram, Features, Electronic Configuration of Sulphur and Nitrogen According to the Aufbau Principle, electrons are filled in order of increasing energies in their ground state. The atomic orbital with the lowest energy level occupies first, compared to higher energy levels. The word Aufbau comes from a German word which means construct or built up. The term n represents the Principal Quantum Number and I is Azimuthal Quantum Number. The Aufbau Principle is used to illustrate the concept of location of an electron in different energy levels. For example, carbon has 6 electrons so its configuration is 1s² 2s²2p². It is important to note that each orbital can hold a maximum of two electrons.

## Drawing the Aufbau Diagram

The following steps show how to draw an Aufbau diagram:

1. Determine the number of electrons in the atom.
2. Fill the s orbital in the first energy level (1s orbital) with the first two electrons.
3. Fill the s orbital in the second energy level (2s orbital) with the second two electrons.
4. Put one electron in each of the three p orbitals in the second energy level (2p orbitals) and then if there are still electrons remaining, go back and place a second electron in each of the 2p orbitals to complete the electron pairs.
5. Continue this way through each of the successive energy levels till all the electrons have been allotted.

## Features of the Aufbau Principle

1. According to the Aufbau Principle, the lowest energy electron occupies first in the position.
2. Energy of orbital increase can be determined from the n + 1 rule. The sum of principal and mutual Quantum numbers determine the different energy levels of the orbitals.
3. The lower n+1 value corresponds to lower orbital energies.If two orbitals share equal n+1 value,then the orbital with lower n value is considered as lower energy.
4. Electrons are filled with electrons in order 1s, 2s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p, 7s, 5f, 6d, 7p.

## Exceptions

There are many exceptions to the Aufbau Principle.
One example is the electronic configuration of chromium is [Ar]3d⁵4s¹ not [Ar] 3d⁴4s². The half-filled subshell shows electron-electron repulsion in the orbital while increasing the stability. Similarly the completely filled subshell increases the stability of the atom.
Many electronic configurations of atoms don't follow Aufbau principle e.g. copper.

## Electronic configuration of Sulphur 1. The atomic number of sulphur is 16. It means it has a total of 16 electrons.
2. According to the Aufbau Principle, two of these electrons are present in 1S subshell 8 are present in 2S and 2P subshell whereas remaining electrons are present in 3s and 3p subshell.
3. The electronic configuration is written as 1s²2s²2p⁶3s²3p⁴.

## Electronic configuration of Nitrogen 1. Nitrogen has 7 electrons.
2. Each electron is filled in the 1s, 2s, 2p orbitals.
3. The electronic configuration of Nitrogen 1s²2s²2p³.

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