A Note on Electronic Configuration of Elements and Stability of Orbitals
Every atom contains a central structure called nucleus which further contains protons and neutrons. The electrons revolve around this nucleus in 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 orbit contains two electrons and the second energy level of s orbit contains one electron.
Following diagram will elucidate this concept.
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Fig1: Electron configuration
The movements of electrons are more complicated than it seems. They always follow some specific pathways which are called orbitals or sub-shells. These orbitals can be found in several different shapes such as s p d f. In that case, first energy level contains only one s orbital whereas second one comprises one s and three p orbitals. Similarly, the third energy level possesses one s three p and five d orbitals. Among all orbitals, s is at the lowest energy.
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Fig 2: Orbital diagram
This orbital diagram determines an atom’s electron configuration. Following are some features of this configuration.
Each sub-shell can contain a maximum of two electrons. An electron pair means electrons that stay together in a sub-shell or orbital.
An electron always enters an orbital having lowest energy.
The electrons have their own 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 a few general rules that help in determining orbital configuration of an atom.
The German word ‘Aufbau’ means building up. According to this law, electrons always enter the orbitals following increasing energy order. It looks something like this.
It can also be expressed with Madelung’s rule
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Fig3: Madelung rule of energy order.
In Aufbau diagram, the electrons are presented as arrows. This principle states that an atom “build-up” gradually by adding electrons. The upward arrows denote an electron is moving in one way, and the downward ones denote that the electron is moving in an opposite direction. For a matter of fact, the orbital having only one electron is called 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 periodic table.
Hund’s rule tells that
At sub-levels, each orbital is occupied singly then any orbital is occupied doubly.
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 paring 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 orbit than sharing one. Furthermore, the calculations of quantum- mechanics, the electrons of orbitals occupied singly are not properly shielded from 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 direction. 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 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.
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Fig4: Oxygen electron configuration following Hund’s rule
Pauli Exclusion Principle
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 always should 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 particle 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 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 sub-shell 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 electron from the outermost p sub-shell, 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 human body.
Test your knowledge
Write the electron configuration of the following
Answers: 1- 1s22s22p63s23p64s23d7;
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1. Which Electron Configuration is the Most Stable?
Ans. It is observed that the noble gases have the most stable orbital configuration. The reason behind it is their valence shell is completely filled. In the case of helium, the two valance electrons exist in 1s sub-level, and other eight electrons are in the s and p sub-shells.
2. How Electron is Arranged in an Orbital?
Ans. The nucleus of an atom is surrounded by orbitals, and on these orbitals, electrons are arranged. However, the electrons closest to the nucleus contain lowest energy, and the furthest ones have the highest energy. Also, the electrons primarily move within orbitals or sub-shells of an atom.
3. How Many Electrons are there in a Shell?
Ans. Each shell is filled with a fixed number of electrons. It is determined by a simple formula, where for nth shell, the maximum number of electrons will be 2(n)2. For example, the first shell holds a maximum of 2 electrons, and for second and third ones, the number goes up to 8 and 18, respectively.
4. What Does Orbitals s p d f Mean?
Ans. Orbital names refer to the group lines originally to the spectra of alkali metals. Thereby these groups of lines s p d f mean sharp, principle, diffuse and fundamental, respectively.