# The Aufbau Principle, Salient Features and Exception.

The Aufbau principle states that in the ground state of an atom or ion, electrons fill atomic orbitals of the lowest available energy level before occupying higher-energy levels. It is a physical principle that states that electrons fill atomic orbitals of the lowest available energy level before occupying higher energy levels.

# What is mean by the Aufbau Principle?

The Aufbau principle states that in the ground state of an atom or ion, electrons fill atomic orbitals of the lowest available energy level before occupying higher-energy levels. It is a physical principle that states that electrons fill atomic orbitals of the lowest available energy level before occupying higher energy levels.

The order of increasing energy of the orbitals is determined by the (n+l) rule, where n is the principal quantum number and l is the angular momentum quantum number. The Aufbau principle is named after the German word “Aufbauprinzip”, which means “building-up principle”. It was formulated by Niels Bohr and Wolfgang Pauli in the early 1920s.

The (n+l) rule states that the energy of an orbital increases with the principal quantum number (n). However, if two orbitals have the same principal quantum number (n), then the orbital with the higher angular momentum quantum number (l) will have a higher energy.

For example, the 2s orbital has a lower energy than the 2p orbital because the 2s orbital has a lower principal quantum number (n). However, the 3d orbital has a higher energy than the 4s orbital because the 3d orbital has a higher angular momentum quantum number (l).

The principal quantum number tells us how far away the orbital is from the nucleus, and the angular momentum quantum number tells us the shape of the orbital.

## The order of increasing energy of the orbitals:

The order of increasing energy of the orbitals is as follows:

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

## The salient features of the Aufbau principle:

The salient features of the Aufbau principle are as follows:

1.Electrons fill atomic orbitals of the lowest energy level first.

2.The energy of an orbital depends on the principal quantum number (n) and the azimuthal quantum number (l).

3.The order of filling of orbitals is determined by the (n+l) rule.

4.An orbital can hold at most two electrons.

5.If two or more orbitals of the same energy are available, electrons fill them singly before pairing up (Hund’s rule).

The Aufbau principle is a fundamental principle of quantum mechanics that is used to predict the electron configuration of atoms. It is based on the idea that electrons are attracted to the nucleus and that they will fill the lowest energy orbitals first.

The (n+l) rule is a way of ranking orbitals by their energy. The (n+l) value of an orbital is the sum of its principal and azimuthal quantum numbers. The orbital with the lowest (n+l) value has the lowest energy.

Hund’s rule states that in degenerate orbitals, electrons fill each orbital singly before pairing up. Degenerate orbitals are orbitals of the same energy. This means that when there are multiple orbitals of the same energy, the electrons will fill each orbital with one electron before any orbitals are filled with two electrons.

The Aufbau principle is a simple and elegant way of predicting the electron configuration of atoms. However, there are a few exceptions to the rule that are caused by the complex interactions of the electrons in an atom.

## Exception:

The Aufbau principle states that electrons fill atomic orbitals of the lowest energy level first. However, there are a few exceptions to this rule.

1.Chromium (Cr): The electron configuration of chromium is 1s2 2s2 2p6 3s2 3p6 4s1 3d5. According to the Aufbau principle, the electron configuration of chromium should be 1s2 2s2 2p6 3s2 3p6 4s2 3d4. However, the 3d orbital is slightly lower in energy than the 4s orbital for chromium. This is because the 3d orbital is more compact and has a higher electron density.

2.Copper (Cu): The electron configuration of copper is 1s2 2s2 2p6 3s2 3p6 4s1 3d10. According to the Aufbau principle, the electron configuration of copper should be 1s2 2s2 2p6 3s2 3p6 4s2 3d9. However, the 3d orbital is more stable when it is filled to capacity. This is because the 3d orbital is more compact and has a higher electron density.

Other exceptions to the Aufbau principle are found in the lanthanide and actinide series of elements. These elements have many electrons in their f-orbitals, which are located in the fourth and fifth energy levels. The f-orbitals are very close in energy, and their order of filling is not well understood.

## Conclusion:

In general, the Aufbau principle is a reliable guide for predicting the electron configuration of atoms. However, there are a few exceptions to the rule that are caused by the complex interactions of the electrons in an atom.The Aufbau principle is one of the three rules that govern the filling of atomic orbitals. The other two rules are the Pauli Exclusion Principle and Hund’s rule.

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