Aufbau Principle

According to the Aufbau Principle, electrons in their ground state are filled in the order of increasing energies, starting with the lowest. When comparing lower energy levels to higher energy levels, the atomic orbital with the lowest energy level takes up the most space. The phrase Aufbau is derived from a German word that literally translates as “construction” or “building up.” The term n refers to the Principal Quantum Number, while the term I refer to the Azimuthal Quantum Number. It is necessary to explain the Aufbau Principle in order to understand the concept of an electron’s position at different energy levels. Carbon, for example, possesses six electrons, resulting in the structure 1s² 2s²2p². It is vital to remember that each electron can only occupy a maximum of two orbitals in the atom.

Filling of electron shells and subshells 

The electrons in the vicinity of an atomic nucleus have discrete energy levels, which are referred to as shells. There are just two electrons in a shell known as the s shell at the lowest energy level, which is closest to the nucleus. This is the lowest possible energy level. The next shell has space for eight electrons in two subshells, the s, and the p subshells, which are located adjacent to one another. The third shell has space for 18 electrons, which are divided into three subshells, which are the s, p, and d subshells. The fourth shell has four subshells, with the f subshell being the final one. The lettered subshells always have the same number of electrons available: two for the s subshell, six for the p subshell, ten for the d subshell, and fourteen for the f subshell. The number of the main shell and the letter of the subshell is used to identify a subshell in order to distinguish it. For example, hydrogen has one electron in the 1s shell, whereas oxygen has eight electrons, two in the 1s shell, two in the 2s subshell, and four in the 2p subshell. Hydrogen has one electron in the 1s shell, while oxygen has two electrons in the 1s shell, two in the 2s subshell, and four in the 2p subshell. After then, the subshells fill up sequentially according to their numbers and letters until the third shell is reached.

The 3s and 3p subshells are completely filled with two and six electrons, respectively, but the next electrons are deposited in the 4s subshell rather than the 3d subshell, as expected. It is because the 4s subshell has a lower energy level than the 3d subshell that it is the first to fill. In spite of the fact that the numbers are out of order, they adhere to the Aufbau principle because the electron subshells fill up in accordance with their respective energy levels.

Steps to draw the Aufbau diagram 

1. Count the number of electrons in the atom to determine its composition.
2. Insert the first two electrons into the s orbital of the first energy level (1s orbital) to complete the filling of the s orbital.
3. Insert the second and third electrons into the s orbital of the second energy level (2s orbital) to complete the filling.
4. Insert one electron into each of the three p orbitals in the second energy level (2p orbitals), and then, if there are still electrons available, insert a second electron into each of the two orbitals to complete the electron pairs. 
5. Follow this procedure through each of the following energy levels until all of the available electrons are used up.

Salient features of Aufbau principle 

1. According to the Aufbau Principle, the electron with the lowest energy takes the first position in the structure.
2. Using the n + 1 rule, it is possible to calculate the energy of orbital growth. The varying energy levels of the orbitals are determined by the sum of the principal and mutual Quantum numbers in the system.
3. Lower orbital energy is associated with lower n+1 values.
4. If two orbitals have the same n+1 value, the orbital with the lower n value is regarded to have the lower energy value.
5. Fourteenth, electrons are filled in the following order: 1s, 2s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p, 7s, 5f, 6d, 7p.

Conclusion

According to the Aufbau Principle, electrons in their ground state are filled in the order of increasing energies. Carbon, for example, possesses six electrons, resulting in the structure 1s2 2s2 2p2. Each electron can only occupy a maximum of two orbitals in the atom. The electron subshells fill up in accordance with their respective energy levels. According to the Aufbau Principle, the electron with the lowest energy takes the first position in the structure. Using the n + 1 rule, it is possible to calculate the energy of orbital growth.