 One of the things we learn in physical and organic chemistry is the state of electron orbitals. We can understand the state of a molecule by first learning how electrons exist.

But as quantum mechanics, the state of the electron is abstract and unusually difficult to understand. For example, there are many people who try to study organic chemistry without a deep understanding of the electron orbital state.

However, if you learn about the electron state, you can understand the phenomenon more deeply. Pauli exclusion principle and Hund’s rules are two examples of these electron states.

Some professors may use difficult mathematical equations to explain Pauli exclusion principle and Hund’s rules. But unless you are a physicist, these formulas are useless. Therefore, we will explain Pauli exclusion principle and Hund’s rules in a simple way without using mathematical formulas and by comparing them with phenomena in the world.

## The Wave Function in Quantum Mechanics Represents the Probability of the Existence of Electrons

Quantum mechanics is the study of the states of electrons, protons and neutrons. Quantum mechanics typically deals with electrons. The field of quantum mechanics has produced several Nobel Prize winners in physics.

In quantum mechanics, there is something called the wave function. What is the wave function? In high school chemistry, we learn about how electrons orbit around atoms like planets. But this diagram is wrong. The electrons do not revolve around the atoms. The electrons are in an orbit, but it is not clear if they exist in an electron orbit. The probability of the electron being in an electron orbit is high, but it is also possible that the electrons could be elsewhere.

The area where electrons exist is called the electron cloud, and the electron orbit is the only place where electrons have a high probability of being in the orbit. So, the wave function tells us how likely the electrons are to exist in that location.

Electrons are a type of wave, hence the name wave function. It is the wave function that tells us the shape of the electron and the probability of its existence.

## Pauli Exclusion Principle Describes the State of the Electron

So what is Pauli exclusion principle? Pauli exclusion principle is a study of quantum mechanics and describes the state of electrons.

Pauli exclusion principle can be very simply stated as follows.

• Only two electrons can enter one orbit.

There are different types of electron orbitals. The s and p orbitals are particularly important in physical and organic chemistry. They look like the following. Electrons have a high probability of existing in these orbitals. However, electrons are not free to enter these orbitals; only a maximum of two electrons can enter a single orbit. This is Pauli exclusion principle.

In 1945, the Swiss physicist Wolfgang Pauli was awarded the Nobel Prize in Physics. He was awarded the Nobel Prize in Physics for his discovery of the Pauli exclusion principle.

### Electrons Have a Spin, and They Have to Face Each Other in Opposite Directions

There are other important ideas in Pauli exclusion principle. It is the orientation of the electron. When the electrons enter the orbit, they can’t be in any direction. They must be facing away from each other to enter the electron orbit. Electrons have a spin. Electrons also have an electric charge. When an electrically charged substance rotates, it produces a magnetic force.

Magnets have an N-pole and an S-pole, and if they go in the same direction, they repel. For example, if you put them close to the N-pole and the N-pole, they will repel. In order to cancel each other out, the electrons do not enter the electron orbit in the same direction, but in the opposite direction.

To be more precise, electrons with the same wave function cannot enter the same orbit. But using the term “wave function” is difficult to understand. So, let’s understand that electrons spinning in the same direction cannot enter the same electron orbit.

### The Case of the Pauli Exclusion Principle

The Pauli exclusion principle is all around us. For example, phosphorescence falls into this category. The following is the path that absorbs the sun’s rays during the day and gives off light at night. Phosphorescence is the phenomenon of continuously emitting weak light, even when we stop shining light on it. Why does phosphorescence, unlike fluorescence, continue to emit light in phosphorescence, even though it is not illuminated? This has to do with Pauli exclusion principle.

When light energy strikes an electron, the electron moves to another orbit. This is called electronic transition. However, the state in which the electron transition occurs is unstable (excited state) and tries to return to the original state (ground state). At this time, the electrons return to the ground state by releasing heat and light. In some materials, however, the electron spin can change direction after the electron transition when light hits the material. In technical terms, this state is called a triplet excited state. According to Pauli exclusion principle, electrons with the same wave function (electron spins) cannot enter the same orbit.

After the electron transition, the direction of the electron spins must be reversed in order to return to the ground state from the unstable excited state. Therefore, the electron cannot return to the ground state immediately from the excited state and emits weak light continuously for a long time. This is the principle of phosphorescence. If we learn Pauli exclusion principle, we can understand why phosphorescence arises. Some of the phenomena around us can be explained by Pauli exclusion principle.

It is known that Pauli exclusion principle applies to not only electrons but also protons and neutrons without exception. Although there are some exceptions, many substances follow the Pauli exclusion principle.

## Hund’s rules to learn about electronic placement

Does an electron orbit always have to have two electrons? Of course not. When two electrons are in the same electrical orbit, the electron density increases. The electrons should be separated from each other because they repel each other.

In such cases, Hund’s rules is used to indicate the order in which the electrons enter the orbit. Hund’s rules is the rule for electron placement. Hund’s rules is as follows.

• When electrons enter an orbit, they enter in order from another orbit.

Each molecule has its own orbitals. If there are orbitals of the same energy, then when an electron enters an orbit, it goes into each orbit one at a time. This is Hund’s rules. Thus, one by one, the electrons go in.

### Electrons Enter from the Lowest Energy Orbitals First

But the electron configuration we just described is incorrect because it doesn’t take into account the s and p orbitals. There are two types of electron orbitals, s-orbital and p-orbitals, and the energy of the s-orbital is lower than that of the p-orbitals.

Even in the same L-shell, the s-orbital has lower energy than the p-orbital. As a result, two electrons go into the s orbital first, and then one into the p orbit. After that, the electrons enter the p orbitals. For carbon, nitrogen and oxygen atoms, Hund’s rules states that they enter the electron orbitals as follows Each atom has the following electrons

• Carbon atom: 6
• Nitrogen atom: 7
• Oxygen atom: 8

If the electrons enter in the order of low energy, the figure above shows the result. Since the energy is lower in the s-orbital, two electrons initially enter the s-orbital. After that, the electrons enter the p-orbital, which has three orbitals with the same energy. Therefore, when the electrons enter the p orbitals, they enter each orbit one by one.

The electron configuration is arranged according to these rules. When the electrons are arranged according to Hund’s rules, an unpaired electron is formed. If the electrons are arranged according to Hund’s rules, the number of unpaired electrons is the maximum.

In any case, this electron configuration of the atoms is Hund’s rules. By learning Hund’s rules, we can understand the electron configuration of atoms, their energies, and the bonds between molecules.

## Understanding the Environment of Electrons in Atoms and Molecules

An important element in chemistry is the state of electrons. If we know what state the electrons are in, we can understand how chemical reactions work.

However, what makes these quantum mechanics difficult is that they are not as clearly discernible as in organic chemistry or biochemistry. For example, in a biological experiment, we can check the state of cells. On the other hand, it is difficult to identify the electrons in atoms and molecules.

Therefore, it is the wave function in quantum mechanics that is difficult to understand. There is Pauli exclusion principle and Hund’s rules in such quantum mechanics.

However, the concepts of Pauli exclusion principle and Hund’s rules are simple if you don’t use difficult mathematical formulas. It is these laws that describe the state and arrangement of electrons.

If you want to know why these laws are in place, you will need to do some difficult calculations. In that case, you will need to study physics in depth as quantum mechanics. However, for many people, there is no point in doing difficult calculations to find out why, so make sure that you understand that these laws exist before you take the next step.