Difference Between Aufbau Principle and Hund’s Rule

Main Difference – Aufbau Principle vs Hund’s Rule

The development of the atomic structure began with Dalton’s modern atomic theory. It stated that all matter is made of atoms and atoms cannot be further divided into smaller particles. However, it was later found that the atom can be further divided into sub-atomic particles after the discovery of the electron by J.J. Thompson, the discovery of the nucleus by Rutherford and the concept of electron orbitals of Niels Bohr. The structure of the atom that is accepted at present includes details about electron shells, subshells, and orbitals. The way electrons fill into these shells and orbitals can be described using the Aufbau principle and the Hund’s rule. The main difference between  Aufbau Principle and Hund’s Rule is that Aufbau principle indicates the order in which subshells are filled with electrons whereas Hund’s rule indicates the order in which orbitals of subshells are filled electrons.

Key Areas Covered

1. What is Aufbau Principle
      – Theory, Explanation with Examples
2. What is Hund’s Rule
      – Theory, Explanation with Examples
3. What are the Similarities Between Aufbau Principle and Hund’s Rule
      – Outline of Common Features
4. What is the Difference Between Aufbau Principle and Hund’s Rule
      – Comparison of Key Differences

Key Terms: Atom, Aufbau Principle, Electron, Hund’s Rule, Orbital

What is Aufbau Principle

Aufbau principle states that the order of filling of electrons into subshells of an atom occurs from the lowest energy level to the highest energy level. In other words, when electrons are filled into the orbitals of an atom, the electrons first fill the orbitals that are in the lowest energy level before filling the high energy levels.  

Generally, the energy increases in the order of 1 < 2 < 3 < 4 in the shell level and s < p < d < f in orbital level. For example, an s, p, d or f orbital in the 2^nd shell should always have lower energy than that of the 3^rd shell. But according to Aufbau principle, electrons are sometimes filled into these orbitals with exceptions. For example, the 4s orbital has low energy than that of the 3d orbital although the subshell 3 comes before subshell 4. Here, the order of filling electrons into orbitals is different from the expected order.

Expected Order

1s < 2s < 3s < 3p < 3d < 4s < 4p < 4d < 5s…

Actual Order

1s < 2s < 3s < 3p < 4s < 3d < 4p < 5s < 4d…

However, it is difficult to remember the energy levels of each orbital one by one. Therefore, we can use the following diagram to determine the energy levels easily.

Figure 1: Order of the Energy Levels of Orbitals

The above image shows a diagram for the determination of energy levels. Here, we can obtain the order of the orbitals by following the path of the arrows. After each arrow head, start with the next arrow. This way, it is easy to obtain the energy levels.

What is Hund’s Rule

Hund’s rule explains the order of the electrons filling into orbitals of subshells. Subshells are composed of orbitals. The number of orbitals present in one subshell is different from one subshell to another. For example, s subshell has only one s-orbital, p subshell has 3 p-orbitals, and d subshell is composed of 5 d-orbitals. Thus, there should be an order of filling these orbitals with electrons. Otherwise, these atoms become unstable.

One orbital can hold a maximum of 2 electrons. According to Hund’s rule, every orbital in the same subshell is first singly occupied by electrons before they pair. This means, electrons are first filled as unpaired electrons and then are coupled. Therefore, when assigning electrons into orbitals, this rule is followed. This is because, if there are orbitals with electron pairs and empty orbitals in the same subshell, it is an unstable configuration since electrons are negatively charged and repel each other when they are in the same orbital. Therefore, electrons tend to arrange in such a way where repulsion among electrons is minimized.

Figure 2: Electrons Occupying Orbitals

Furthermore, this rule explains that electrons are filled to orbitals in such a way to match their “spin.” In other words, electrons in singly occupied orbitals of the same subshell have the same spin. When these electrons are paired, two electrons have opposite spins in order to minimize the repulsion between them. One electron of the electron pair “spin-up” whereas the other electron “spin-down.”

Figure 3: Spin of Electrons in Orbitals

If an orbital is singly occupied, that electron can be either “spin-up” or “spin-down.” However, when that electron is paired, the other electron should have the opposite spin from this electron. This way, the repulsion is minimized.  

Similarities Between Aufbau Principle and Hund’s Rule

• Both Aufbau principle and the Hund’s rule indicates the order in which energy levels of an atom are filled with electrons.

Difference Between Aufbau Principle and Hund’s Rule

Definition

Aufbau Principle: Aufbau principle explains the order in which subshells of an atom are filled with electrons.  

Hund’s Rule: Hund’s rule explains the order in which orbitals of subshells are filled electrons.

Theory

Aufbau Principle:  According to Aufbau principle,  subshells are filled from the lowest energy level to the highest energy level.

Hund’s Rule: According to Hund’s rule, orbitals are first singly occupied by electrons, then they are paired according to their spins.

Energy Levels

Aufbau Principle: Aufbau principle describes the way electrons fill subshells.

Hund’s Rule: Hund’s rule describes the way electrons fill orbitals of subshells.

Repulsions

Aufbau Principle: Aufbau principle does not describe the minimization of repulsions between electrons.

Hund’s Rule: Hund’s rule indicates how electrons are filled in a way to minimizes the repulsions between electrons.

Conclusion

Both Aufbau principle and Hund’s rule are very important in developing the atomic structure of a particular atom. If the number of electrons is known for a certain atom, then we can determine the pattern that these electrons are arranged in that atom with the use of above theories. The main difference between the Aufbau principle and the Hund’s rule is that Aufbau principle indicates the order in which subshells are filled with electrons whereas Hund’s rule indicates the way in which electrons fill the orbitals in the subshells.

References:

1.”Aufbau principle.” Wikipedia. Wikimedia Foundation, 21 July 2017. Web. Available here. 02 Aug. 2017. 
2.”Hunds Rules.” Chemistry LibreTexts. Libretexts, 21 July 2016. Web. Available here. 02 Aug. 2017. 

Image Courtesy:

1. “Klechkovski rule” By Bono~commonswiki assumed (based on copyright claims) (CC BY-SA 3.0) via Commons Wikimedia
2. “Orbital diagram nitrogen – Hund’s Rule” Por CK-12 Foundation (raster), Adrignola (vector) – File:High School Chemistry.pdf, page 325 (Domínio público) via Commons Wikimedia
3. “Hund’s Rule” By CK-12 Foundation (raster), Adrignola (vector) – File:High School Chemistry.pdf, page 323, (Public Domain) via Commons Wikimedia