Electronic Configuration of Elements and Stability of Orbitals

Science is a spectacular subject that includes application based learning of each topic to understand and execute what is learnt in real life. This makes it fun for students and provides an intensive learning of our environment as a whole. Chemistry is one of the branches of science that deals with substances and their chemical properties and reactions in different areas of nature. 

Every atom contains a central structure called nucleus which further contains protons and neutrons. The electrons revolve around this nucleus in a certain axis. These electrons are arranged in different shells that surround the nucleus.

If you compare it with an onion, the centre of it is a nucleus, and the layers are the shells. Moreover, just like an onion, the shells also have a layered arrangement. Furthermore, each shell has a specific number of electrons that orbit around the nucleus. 

Therefore, the orbital configuration refers to a layered arrangement of shells, where a specific number of electrons orbit around the nucleus. 

Standard Notation of Electron Configuration

A typical 2n2 notation is used to show the electron configuration. It involves the energy level, orbital number and the number of electrons. For example, lithium’s configuration is 1s22s1. It means the first energy level of s orbital contains two electrons and the second energy level of s orbit contains one electron. 

Atomic Orbitals

The movements of electrons are more complicated than it seems. They always follow some specific pathways which are called orbitals or subshells. These orbitals can be found in several different shapes such as s p df. In that case, the first energy level contains only one s orbital whereas the second one comprises one s and three p orbitals. Similarly, the third energy level possesses three p and five d orbitals. Among all orbitals, s is at the lowest energy. 

This orbital diagram determines an atom’s electron configuration. Following are some features of this configuration. 

• Each subshell can contain a maximum of two electrons. An electron pair means electrons that stay together in a subshell or orbital. 

• An electron always enters an orbital having the lowest energy. 

• The electrons have their orbital filling mechanism. It suggests that electrons will always fill an s orbital and then will proceed with p orbital. 

• For example, s orbital holds 2 electrons, while p holds 6. 

This configuration helps in rationalising chemical properties and also defines atomic spectra. 

Electron Configuration Rules

There are a few general rules that help in determining the orbital configuration of an atom. 

Aufbau Principle

The German word ‘Aufbau’ means building up. According to this law, electrons always enter the orbitals following an increasing energy order. It looks something like this. 

1s < 2s < 2p < 3s < 3p < 4s < 3d < 4p < 5s < 4d < 5p < 6s < 4f < 5d < 6p < 7s < 5f < 6d < 7p

It can also be expressed with Madelung’s rule 

In the Aufbau diagram, the electrons are presented as arrows. This principle states that an atom “build-up” gradually by adding electrons. The upward arrows denote that an electron is moving in one way, and the downward ones denote that the electron is moving in an opposite direction. The orbital having only one electron is called an unpaired electron.   

However, there is a limitation of this principle as it states that the orbital energy orders are fixed. But it is not always physically reasonable. Because of that, it is useful for the first 18 elements of the periodic table. 

Hund’s Rule

Hund’s rule tells that

1. At sub-levels, each orbital is occupied singly then any orbital is occupied doubly. 

2. The electrons that occupy an orbital singly follow the same spin. 

In simple words, the first rule states that electrons always fill an empty orbital before pairing up. As we know, electrons have a negative charge, and therefore they repel one another. Hence, to avoid this repulsion, they try to occupy individual orbits rather than sharing one. Furthermore, in the calculations of quantum- mechanics, the electrons of orbitals occupied singly are not properly shielded from the nucleus. 

The second rule justifies that if all the electrons spin in the same direction, they will encounter each other more often than the ones spinning in opposite directions. If they often meet, the repulsion force will increase and set them apart. Because of this fact, all paired electrons contain lower energy than the unpaired ones. 

Following Hund’s rule, in a sub-level, the first electron can be “spin up” as well as “spin down”. However, for the other electrons, the spin will be the opposite of the former one. Generally, scientists mark the first electron and the unpaired ones as spin-up and then place the other ones accordingly, to avoid mistakes. 

For example, refer to the configuration of oxygen which has 8 electrons. 

Pauli Exclusion Principle

The Pauli Exclusion Principle was proposed by Wolfgang Pauli, an Australian physicist. According to this principle, two electrons of any single atom cannot have the same quantum numbers. In simple words, every electron should always be in its unique state of singlet state. It follows the below-mentioned rules.

• Only two electrons can be in a single orbit. 

• The electrons present in the same orbitals should follow opposite spin. 

However, this principle is not limited only to electrons but also to fermions or particles with half-integer spin. Therefore, when two electrons fill the same orbital, for example, the 3s orbital, spins have to be paired. 

Following the rules mentioned above, we can comprehend the orbital arrangement of electrons.  

What is a Stable Electron Configuration?

Electron configuration can predict the stability of an atom. When an atom’s all orbitals are filled, it becomes the most stable, thereby unreactive. Moreover, the most stable electron configuration possesses a full state of energy. These orbital configurations are major characteristics of noble gases; therefore, they do not react with other compounds easily. 

It is the valence shells or the outermost electrons of an atom that interact first when two or more atoms come in contact with each other. When an atom’s valence shell is filled, it becomes the most stable. These valence electrons decide the chemical behaviour of an element. For example, the elements having the same valence electrons show similar chemical properties. 

Configuration of Anions and Cations

Anions’ electron configuration involves the addition of electrons following the Aufbau principle. First, we place an electron to occupy the outermost subshell and then proceed to fill the next higher orbital. For instance, the neutral atom, chlorine, contains 17 electrons. Therefore, we can write the configuration as 1s22s22p63s23p5.

Similarly, Chloride ion or Cl- contains 18 electrons. Therefore, the single electron will occupy first the partially filled 3p orbital, filling it completely. Hence, we can write the configuration as 1s22s22p63s23p6.

In case of Cations, the electron configuration will be drawn first by eliminating electrons from the outermost p subshell, then from s and d orbital as well. For example, the electron configuration of calcium is 1s22s22p63s23p64s2. However, the calcium ion or Ca2+ has two less electrons. Hence the configuration of it will be 1s22s22p63s23p6. 

Similarly, you can write the orbital configuration of several other atoms as well to get accustomed to the methods.  

Thus, electron configuration plays a vital role in determining the behaviour or chemical properties of an element, starting from the simplest molecules like helium and hydrogen to most complex ones that are seen in the human body.  

Test your knowledge

Write the electron configuration of the following 

1. Zn3+

2. P

Answers: 1- 1s22s22p63s23p64s23d7; 

                  2- 1s22s22p63s23p3

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