Hunds Rule

Electronic Configuration - Pauli's Exclusion Principle, Aufbau Principle and Hunds Rule


Electrons are small compared to protons and neutrons, over 1,800 times less than either a proton or a neutron. Electrons have a relative mass of 0.0005439 such that the electron is compared with the mass of a neutron being one or about 9.109x10-31 kg.

The electron was discovered in the year 1897 by a British physicist J.J. Thomason. Basically known as "corpuscles," electrons have a negative (-ve) charge and are electrically pulled to the positively charged protons. The atomic nucleus is surrounded by electrons in pathways called orbitals, this idea was put forth by Erwin Schrödinger, a physicist, in the 1920s. Now, this model is known as the quantum model or the electron cloud system. The inner orbitals surrounding the atom are Spherical, but the outer orbitals are much more complex.

An atom's electron configuration is the orbital description of the positions of the electrons in a typical atom. Using the electron configuration and laws of physics, chemists can predict an atom's properties, such as stability, boiling point, and conductivity.

Electronic configuration

Method or distribution of electrons in the orbitals of an atom. Atom comprises of sub-atomic particles like electrons, protons, and neutrons among which only the number of electrons is considered for electronic configuration. Electrons are supplied in such a way that they achieve a high constant configuration.

The atom consists of s, p, d, and f orbitals in which s orbital can hold a maximum of 2 electrons in them,
The p orbital can hold a maximum number of 6 electrons, 'd' orbital can hold a maximum number of 10 electrons and f orbitals can hold a maximum number of 14 electrons in the orbital shell.

For example: Chlorine 17


In the above distribution of electrons orbitals in which s orbital can hold a maximum of 2 electrons in them, The p orbital can hold a maximum number of 6 electrons


The electron shells are named by K, L, M, N, O, P, and Q; or by 1, 2, 3, 4, 5, 6, and 7; going from innermost shell to outmost shell. Every shell is formed by one or more subshells, which are formed by the composed of atomic orbitals it is called as the subshell

Electron Spin:

Electron spin is a quantum feature of electrons. It is a kind of angular momentum. The magnitude value of this angular momentum is permanent. Like charge and rest mass, spin is a basic, unvarying property of the electron.

As a teaching method, we can sometimes liken electron spin to the earth spinning on its own axis every 24 hours. If the electron spins clockwise on its axis, it is called as spin-up and it is counterclockwise then it is called spin-down. This is a suitable explanation, if not fully justifiable mathematically.

The spin angular momentum linked with electron spin is independent of orbital angular momentum, which is associated with the electrons that travel around the nucleus.


They are three important laws which fulfill these electrons namely

  • 1. Pauli's exclusion principle

  • 2. Aufbau principle

  • 3. Hund's rule.

  • Pauli's exclusion principle

    According to this law, an orbital cannot have both the electrons in the same spin motion (half-integer spin); electrons will be in either positive half spin (+1/2) or negative half spin (-1/2)

    For example, argon's electron configuration:

    1s2 2s2 2p6 3s2 3p6

    The 1s level can accommodate two electrons with same n, l, and ml quantum numbers. Argon's pair of electrons in the 1s orbital meet the exclusion principle because they have opposite spins, determining they have different spin quantum numbers, ms. One spin is +½, the other is -½. (Instead of saying +½ or -½ often the electrons are said to be spin-up up arrow or spin-down down arrow.)

    The 2s level electrons have a separate principal quantum number to these in the 1s orbital. A couple of 2s electrons differ from each other because they have different spins.

    The 2p level electrons have a different orbital angular impulse number from those in the s orbitals, hence the letter p rather than s. There are three p orbitals of similar energy, px, py, and pz. These orbitals are different from one another because they have different bearings in place. Each of the px, py and pz orbitals can contain a pair of electrons with opposite spins.

    The 3s level rises to a greater principal quantum number; this orbital accommodates an electron pair with opposite spins.

    The 3p level's information is similar to that for 2p, but the principal quantum number is higher: 3p lies at a higher energy than 2p.

    Aufbau principle

    This principle explains filling up electrons in rising orbital energy.

    for example

    1s orbital should be fulfilled before 2s orbital for 1s is lower in energy than 2s orbital.

    By regarding these three rules, the electron configuration of an atom is composed.

    for example

    The electron configuration of Carbon atom

    Carbon is a p block element which includes 6 electrons. It comprises s and p orbitals. Hence by grasping the three rules the electronic configuration of carbon atom can be written as

    The electron configuration for carbon atom is recorded as. Total no of 6 electrons is disposed over 1s, 2s, and 2p orbitals. s orbitals can hold two electrons and p orbital holds 2 electrons by following Hund's rule of highest multiplicity.

    Hund's rule.

    According to this principle, for a given electronic configuration, the paring of the particle is done after each subshell is filled with a single electron. In other words, the under subshell should have maximum multiplicity.

    Hund's rule states that:

  • • each orbital in a subshell is only obtained before any orbital is double involved.

  • • All of the electrons in singly occupied orbitals have a similar spin (to maximize total spin).

  • When allowing electrons to orbitals, an electron first seeks to fill all the orbitals with comparable energy (also called to as degenerate orbitals) before joining with another electron in a half-filled orbital. Atoms at ground states tend to have as many unpaired electrons as likely. In reflecting this process, consider how electrons show the same behavior as the same poles on an attraction would if they came into contact with each other; as the negatively charged electrons fill orbitals, they first try to get as far as possible from each other before having to match up.

    According to the first principle, electrons always start with an empty orbital before they join up. Electrons are negatively charged and, as a result, they resist each other. Electrons tend to reduce objection by occupying their own orbitals, rather than receiving or accepting an orbital with another electron.

    Furthermore, quantum-mechanical computations have shown that the electrons in only filled orbitals are small adequately screened or shielded from the nucleus. Electron shielding is further discussed at the next level.
    For the second principle, unpaired electrons in only filled orbitals have similar spins. Technically speaking, the first electron in a sub-shell could be either "spin-up" or "spin-down."

    Once the spin of the first electron in a sub-shell is chosen, however, the spins of all of the separate electrons in that sub-shell depend on that first spin. To avoid interference, scientists typically draw the first electron, and any other unpaired electron, in an orbital as "spin-up."

    For example :


    considering the electron configuration for carbon atoms: 1s22s22p2: The two 2s electrons will fill the similar orbital, whereas the two 2p electrons will be in various orbital (and aligned in the same direction) in accordance with Hund's rule.

    Considering also the electron configuration of oxygen. Oxygen has 8 electrons. The electron configuration can be written as 1s22s22p4. To draw the orbital diagram, begin with the subsequent observations: the first two electrons will pair up in the 1s orbital shell; the next two electrons will pair up in the 2s orbital shell. That leaves 4 electrons, which must be placed in the 2p orbital shell. According to Hund’s rule, all orbitals will be once filled before any electron is double filled. Therefore, two p orbital get one electron and one will have 2 electrons. Hund's rule also specifies that all of the unpaired electrons must have the same spin. In keeping with practice, the unpaired electrons are drawn as "spin-up"

    Answer the following question:

  • 1. what is an electron?

  • 2. Define the term electron spin?

  • 3. State the Hund's rule?

  • 4. Explain Pauli’s exclusion principle?

  • Fill in the blanks:

  • 1. The electron was discovered by __________, a British physicist in the year 1897. (Ans: J.J. Thomason)

  • 2. Every shell is formed by one or more subshells, which are formed by the composed of atomic orbitals it is called as ________ (Ans: subshell)

  • 3. Unpaired electrons in only filled orbitals have similar spins. Technically speaking, the first electron in a sub-shell could be either ______________. (Ans: "spin-up" or "spin-down.")

  • 4. The p orbital can hold a maximum number of ___ electrons, 'd' orbital can hold a maximum number of __ electrons and f orbitals can hold a maximum number of _____ electrons in the orbital shell. (Ans: 6, 10, 14)