Aufbau Principle

Aufbau Principle is the basic method that tells us how the electrons are filled in the atomic orbital. According to the Aufbau Principle, the filling of the electrons in the ground state of an atom follow a basic pattern. This principle tells us that the electrons in the atomic orbital of any atoms are filled in the ascending order of their energy and for finding the energy of the atomic orbital they follow the “n+l” rule.

In this article, we will learn about Aufbau Principle, Features of the Aufbau Principle, its exceptions and limitations in detail.

Aufbau Principle Definition

Aufbau’s Principle states that

“Electrons in different orbitals are filled in the increasing order of their energy, i.e. the orbital with the lowest energy will be filled first and the orbital with the highest energy will be filled last.”

The word ‘Aufbau’ in German means ‘Building up’. Formation of orbitals means filling orbitals with electrons. It states that the electrons in atomic orbitals are filled in the ascending order of the energy of the atomic orbital. And the energy of the atomic orbital is calculated using the “n+l” rule that states if the electron with a higher number of the n+l value has the higher energy. In the rule discussed in the previous line, n tells the Principal Quantum number and l tells the Azimuthal Quantum number.

If two electrons have the same value of “n+l” then the electron with a smaller value of n has the smaller energy and is filled first. The order of filling the electrons in an atomic orbital is discussed below.

Aufbau Principle Diagram

The Aufbau Principle Diagram is the diagram that depicts the order of filling the electrons in an atom.

Aufbau Principle Diagram

Aufbau Principle Formula

The Aufbau Principle Formula is the formula that tells the order in which the electrons are filled in the atomic orbital. The atomic orbital formula is

1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p, 7s, 5f, 6d, 7p…….

Aufbau Principle Class 11

Aufbau Principle is an important principle of Chemistry for the students in class 11. This principle states that,

“The electrons in the atomic orbital of an atom is filled in the ascending order of the energy of the atomic orbital that is calculated using (n+l) rule.”

Drawing the Aufbau Diagram

Aufbau Diagram is drawn using the steps added below,

• Find the number of electrons that is to be filled in the atoms.
• We know that in an atom the
  • s-orbital can hold a maximum of 2 electrons.
  • p-orbital can hold a maximum of 6 electrons.
  • d-orbital can hold a maximum of 10 electrons.
  • f-orbital can hold a maximum of 14 electrons.
• Fill in the first two electrons in the 1s orbital.
• Then follow the “n+l” rule and then fill the electrons accordingly.

Salient Features of Aufbau Principle

Various features of the Aufbau Principle are,

• The energy of an orbital depends on the sum of the values of the principal quantum number (n) and the azimuthal sum number (I). This is called the (n + l) rule. According to this rule,
• In a neutral isolated atom, the lower the value of (n + l) for an orbital, the lower its energy. However, if two different types of orbitals have the same value (n + l), then the orbitals with a lower value of n have less energy.

Type of Orbital	Value of n	Value of l	Value of n+l	Relative Energy
1s	1	0	1+0 = 1	Lowest energy
2s	2	0	2+0 = 2	Higher energy than 1s orbital
2p 3s	2 3	1 0	2+1 = 3 3+0 = 3	2p orbitals(n=2) have lower energy than 3s(n=3) orbital.
3p 4s	3 4	1 0	3+1 = 4 4+0 = 4	3p orbitals(n=3) have lower energy than 4s(n=4) orbitals.
3d 4p	3 4	2 1	3+2 = 5 4+1 = 5	3d orbitals(n=3) have lower energy than 4p(n=4) orbitals.

From the description given in the table, it follows that the energies of the various orbitals increase with the increase in the number of the principal quantum number.

• Thus the order in which the electrons are filled in the atomic orbital of the atom is, 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p, 7s, 5f, 6d, 7p…

Electronic Configuration Using Aufbau Principle

The electronic configuration using the Aufbau Principle for a few elements is discussed below:

Electron Configuration of Sulphur

Atomic Number of Sulphur is 16, i.e. it has 16 electrons

Now using the Aufbau Principle the electronic configuration of the Sulphur is written as,

[S] = 1s² 2s² 2p⁶ 3s² 3p⁴

Electron Configuration of Nitrogen

Atomic Number of Nitrogen is 7, i.e. it has 7 electrons

Now using the Aufbau Principle the electronic configuration of the Nitrogen is written as,

[N] = 1s² 2s² 2p³

Exceptions  of Aufbau Principle

There are a few exceptions to the Aufbau Principle which are Chromium and Copper. Their electronic configuration is,

Electronic Configuration of Chromium

The electronic configuration of Chromium according to the Aufbau Principle should be

• Cr(24): [Ar]3d⁴4s²

But the above configuration is not correct because the actual electronic configuration of Chromium(Cr) is,

• Cr(24): [Ar]3d⁵4s¹

The reason for this electronic configuration is that the half-filled d-atomic orbital is more stable than the orbital filled according to the Aufbau Principle.

Electronic Configuration of Copper

The electronic configuration of Copper according to the Aufbau Principle should be,

• Cu(29): [Ar]3d⁹4s²

The above configuration is not correct because the actual electronic configuration of Copper(Cu) is,

• Cu(29): [Ar]3d¹⁰4s¹

The reason for this electronic configuration is that the full-filled d-atomic orbital is more stable than the orbital filled according to the Aufbau Principle.

Read More

• Quantum Numbers
• Shape of Atomic Orbitals
• Atomic Structure

Aufbau Principle – FAQs

What is Aufbau Principle in Chemistry?

The Aufbau Principle in Chemistry is the basic principle that is used to fill the electrons in the atomic orbitals of any atom. This theorem tells us the order in which the electrons are filled in the atomic orbital.

Who Gave Aufbau Principle?

The Aufbau Principle was given by the famous Danish physicist Niels Bohr in 1920.

What is Pauli’s Exclusion Principle?

The Pauli’s Exclusion Principle states that in an atom no two electrons have all four quantum number the same. Atleast, one of the quantum number is different for all electrons in the atom.

Q4: What is “n+l” rule?

The “n+l” rule is the rule that states the energy of an orbital  depends on the sum of Principal Quantum Number(n) and the Azimuthal Quantum Number(l). this rule is called the (n+l) rule.

Q5: What are the Exception to the Aufbau Principle?

The Exception to the Aufbau Principle are,

• Electronic Configuration of Chromium Cr[24]
• Electronic Configuration of Copper Cu[29]