What is Aufbau’s principle regarding the electron arrangement of atoms?

The Aufbau principle regarding the structure of atoms is that electrons will first enter the lower energy orbitals and gradually enter the higher energy orbitals. Due to the greater stability in the lower energy levels, the electron follows such a continuum in entering orbitals.

The formula for determining the energy of an orbital in Aufbau’s principle is-
E = n + l
Where n is the principal energy level number or principal quantum number of any orbital.
l is the value of the associated quantum number of the energy level. which-
• For s orbitals, l = 0
• For p orbital, l = 1
• For d orbital, l = 2
• For f orbitals, l = 3
E.g. If we determine the energy of 3s orbital-
For 3s, n = 3 and l = 0
Then the energy, E = n + l = 3 + 0 = 3
Similarly, for 4d, E = n + l = 4 + 2 = 6

We have a total of 7 main orbitals for the electron arrangement of all the elements in the periodic table. Among them-
• For the Kth energy level, n = 1; which has orbital – 1s
• For the Lth energy level, n = 2; which has orbitals – 2s 2p
• For the Mth energy level, n = 3; which has orbitals – 3s 3p 3d
• For the Nth energy level, n = 4; which has orbitals – 4s 4p 4d 4f
• For the Oth energy level, n = 5; which has orbitals – 5s 5p 5d 5f
• For P the energy level, n = 6 ; which has orbitals – 6s 6p 6d 6f
• For the Qth energy level, n = 7; which have orbitals – 7s 7p 7d 7f
If we arrange them in order of strength, the order we get is-
1s < 2s < 2p < 3s < 3p < 4s < 3d < 4p < 5s < 4d < 5p < 6s < 4f < 5d < 6p < 7s < 5f < 6d < 7p

However, if two energy levels are equal in energy value, then the energy level in which the value of n is lower, the electron will enter first.
• For example-
• Energy for 4s, E = 4
• Energy for 3p, E = 4
The energy of these two orbitals is equal. So which one will the electron enter first? At 3p! Because 4s and 3p have equal strength. Since the n value of the 3p orbital is low (n = 3), the electron will enter the 3p orbital first.

Even if we arrange the electrons according to the order of energy, we have to keep one more thing in mind.

• When an orbital is filled with electrons, it is the most stable.
• When an orbital is half-filled with electrons, it is fairly stable.
• If an orbital is neither filled nor unfilled with electrons, it is the least stable.
Since the inner energy levels are full, we will try to fill or half-fill the orbital in the outermost energy electron configuration.

Many may think that Aufbau is the name of a scientist or researcher. But no! Aufbau is a German word. Which means – to build or rise.