The Aufbau Principle

The Aufbau principle is one of the most significant principles which determine how the electron shells are built around an atom and filled with electrons. 

The Aufbau principle dictates that electrons will occupy the orbitals, which start with the subshell of the lowest energy. It presents and explains how the electrons fill the atomic orbitals of the atom starting from the ground state. 

According to the principle, electrons start filling the atomic orbitals starting from the lowest energy level and continue in the increasing order of orbital energy level. The Aufbau principle explains the electronic configuration and arrangement of the electrons in the shells, subshells, and orbitals.

Orbital Energy and Quantum Number 

The Aufbau principle dictates that electrons will occupy the orbitals of the atom. However, it is crucial to understand the meaning of energy orbitals. The energy of the orbital is the energy shift necessary for shifting an electron from its orbital to infinity or vice versa. Electronic configuration explains the distribution of the electrons in the atomic orbitals. 

There are a total of four quantum numbers which help in explaining how orbital energy works and determine the electronic configuration: 

• The principal quantum number helps in indirectly describing the size of the electron orbital. It is represented by n. As the value of n increases, the electron gets to a higher energy level and is bound tightly to the nucleus. This shows that electron orbitals show energy levels based on their distance from the atom nucleus. 

• The azimuthal quantum number for an atomic number determines the shape of the orbital. The energy of an orbital is calculated by the sum of the principal and the azimuthal quantum numbers. The azimuthal quantum number is represented by l. 

• The magnetic quantum number helps in distinguishing the orbitals in the subshell of the electron orbital.

• The spin quantum number helps with calculating or determining the intrinsic angular momentum of an electron.

Important Features of the Aufbau Principle 

The Aufbau principle dictates that the electrons will occupy the orbitals and, therefore, the overall electronic configuration of an atom. 

Here are some important features of the principle:

• The Aufbau principle dictates that electrons first occupy the orbitals which have the lowest energy levels. Hence, the electrons enter the orbitals starting from the ground level which is the lowest energy level and move towards higher energy orbitals. 

• The rule for energy orbitals is shown and determined by the combination of both the principal and azimuthal quantum numbers. The energy of an orbital is calculated by the sum of the principal and the azimuthal quantum numbers (n + l). The lower energy level of the atomic orbital is shown by a lower (n + l) value. When two orbitals have equal values for the sum of n and l, the orbital which has a lower n value is considered to have a lower energy level. 

• The first shell of the atomic orbital is shown by 1s. Hence, the electrons are filled in the orbital in the order of 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, and so on. 

Hence, the electrons are filled in a systematic order from lower to high energy levels. The Aufbau principle dictates that the electrons will occupy the orbitals in a sequential way in terms of the level of energy.

Understanding the Aufbau Principle Through Examples 

Each atomic orbital consists of two electrons. The arrangement and distribution of the electrons around the orbital shell and subshell are known as the electronic configuration of an atom. 

Electronic configuration of nitrogen: 

• Nitrogen’s atomic number is 7, which means it has a total of 7 electrons.

• Hence, the electrons are filled in 1s, 2s, and 2p orbitals and hence the electronic configuration for nitrogen is 1s22s22p3.

Electronic configuration of calcium: 

• The atomic number of calcium is 20, and so it consists of 20 electrons.

• Hence, the electrons are filled in 1s, 2s, 2p, 3s, 3p, 4s, and hence the electronic configuration for calcium is 1s22s22p63s23p64s2. 

Exception in the Aufbau Principle

The only element for which there is an exception is the case of chromium. The Aufbau principle dictates that electrons will occupy the orbitals in chromium but there is an exception. As per the Aufbau principle, the electronic configuration of chromium is [Ar]3d54s1 instead of [Ar]3d44s2. This is the only exception due to factors like an increase in the stability due to the half-filled subshells. Another factor that contributes to the exception is the low energy gap between 3d and 4s subshells.

Conclusion 

Aufbau’s principle helps in understanding the electronic configuration and how the electrons fill the orbitals around the atom. The first orbital has the lowest energy or is at ground level. The Aufbau principle dictates that electrons will occupy the orbitals from the ground energy level, and gradually, the electrons will move to a higher energy level. In summary, the first electrons fill the 1s orbital, and each of the orbitals accommodate a maximum of 2 electrons. 

The principle helps in understanding how an electron orbital works. The principal quantum number and the azimuthal quantum number together help in understanding the electron orbital. The energy of an orbital is calculated by the sum of the principal and the azimuthal quantum numbers.