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Electronic Configuration and Diagonal Relationship for JEE

Last updated date: 05th Dec 2023
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What is Electronic Configuration?

In Atomic Physics and Quantum Chemistry, electronic configuration is the distribution of electrons in an atom or molecule (or other chemical entity). For example, the electron configuration of a neon atom is 1s2 2s2 2p6. This means that the 1s, 2s, and 2p subshells are occupied by 2, 2, and 6 electrons, respectively.

Moving diagonally through the periodic table, the elements show certain similarities, but these are far less pronounced than the similarities within the group. The diagonal relationship is especially noticeable in the elements of the 2nd and 3rd periods of the Periodic Table.

The electron configuration of the element represents how the electrons are distributed in their atomic orbitals. The electron configuration of an atom follows the standard notation in which an atomic subshell containing all electrons (including the number of electrons in the superscript) is arranged in sequence. For example, the electron configuration of sodium is 1s2 2s2 2p6 3s1.

Electronic Configuration of Elements

  • Shell

The maximum number of electrons that can be accommodated in a shell depends on the principal quantum number (n). This is expressed by the equation 2n2, where "n" is the shell number. The shell, the value of n, and the total number of electrons that can be accommodated are shown below.

Shell and ‘n’ value

Maximum Number of Electrons that a Shell Can Have

K shell, n = 1 

2 * 12 = 2

L shell, n = 2 

2 * 22 = 8

M shell, n = 3

2 * 32 = 18

N-shell, n = 4 

2 * 42 = 32

  • Subshell

The subshell to which the electrons are distributed is based on the orbital angular momentum (indicated by "l"). This quantum number depends on the value of the principal quantum number n. Therefore, if the value of n is 4, there are 4 different subshells possible.

Subshells correspond to l = 0, l = 1, l = 2, and l = 3 and are called s, p, d, and f subshells, respectively. The maximum number of electrons that a subshell can contain is given by equation 2 ✕ (2l + 1). Therefore, the s, p, d, and f subshells can hold up to 2, 6, 10, and 14 electrons, respectively. Below are all possible subshells with values ​​from n=1 to 4.

Principal and Azimuthal Quantum Numbers

Value of Principal Quantum Number

Azimuthal Quantum Number Value

Subshell in the Electron Configuration

n = 1

I = 0


n = 2

I = 0


I = 1


n = 3 

I = 0


I = 1


I = 2


n = 4

I = 0


I = 1


I = 2


I = 3


It is understood that the 1p, 2d, and 3f orbitals do not exist because the orbital angular momentum value is always smaller than the Principal quantum number.

  • Notation 

The electron configuration of atoms is described using subshell labels. These labels contain the shell number (indicated by the principal quantum number), the subshell name (indicated by the orbital angular momentum), and the total number of electrons in the superscripted subshell.

For example, if two electrons are filled in the "s" subshell of the first shell, the resulting notation would be "1s2".

  • Atomic Orbital Filling

Structural Principle. This principle is named after the German word Aufbeen. This means accumulating. The structural principle shows that electrons occupy low energy orbitals before they occupy high energy orbitals. The energy of the orbit is calculated from the sum of the principal quantum number and the azimuthal quantum number.

According to this principle, the electrons are filled in the following order: 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p, 7s, 5f. , 6d, 7p…

Order of filling Electrons in Atomic Orbital

Order of Filling Electrons in Atomic Orbital 

It is important to note that there are many exceptions to structural principles, such as chromium (1s2 2s2 2p6 3s2 3p6 3d5 4s1) and copper (1s2 2s2 2p6 3s2 3p6 3d10 4s1). These exceptions may be explained by the stability provided by the half-fill or full-fill partial shell.

What is a Diagonal Relationship?

There is a diagonal relationship between the s-block and p-block elements between the 2nd and 3rd periods of the periodic table. The properties of first elements are significantly different compared to the other elements in the group to which they belong.

Diagonal Relationship Examples

The diagonal pairs are:

Group IA Lithium (Li) and Group IIA Magnesium (Mg),

Group IIA Beryllium (Be) and Group IIIA Aluminium (Al),

Group IIIA Boron (B) and Group IVA Silicon (Si), and

Group IVA Carbon (C) and Group VA Phosphorus (P).

Diagonally Related Pair of Elements

Diagonal relationship between lithium and magnesium is mainly due to the following:

  • Li and Mg have about the same electronegativity (electronegativity of Li = 1.0 and electronegativity of Mg is 1.2).

  • Nearly the same radius (radius of Li=152 pm and radius of Mg=160 pm/ionic radius  of  Li+ = 76 pm and ionic radius Mg2+= 72 pm).

Some of the similarities between lithium and magnesium are listed below:

They are harder and lighter than the other elements of their group.

Hydroxides LiOH and Mg(OH)2 are weak bases and decompose when heated.

2LiOH → LiO2 + H2O

Mg(OH)2 → MgO + H2O

Chloride LiCl and MgCl2 are deliquescent and crystallise from the aqueous hydrate solution. These chlorides are also soluble in ethanol. Hydroxides, carbonates, phosphates, and fluorides of Li and Mg are slightly soluble in water.


Electron configuration is the distribution of electrons in an atom or molecule (or other chemical entity). The electron configuration describes each electron as moving independently in the orbit, in the average field generated by all other orbits. The diagonal elements on the second and third row of the Periodic Table show the similarity of the properties. The cause of such similarity and the pair of elements that indicate this relationship.

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FAQs on Electronic Configuration and Diagonal Relationship for JEE

1. What are the applications of electronic configuration?

The most popular application of electron configuration is the rationalisation of chemical properties in both inorganic and organic Chemistry. In fact, electron configuration, along with a simplified form of molecular orbital theory, is a modern equivalent of the concept of valence, which explains the number and types of chemical bonds that an atom can form. To accurately describe a multi-electron system, a large number of electron configurations are required, and energy cannot be allocated to a single configuration. The basic use of electron configuration is the interpretation of atomic spectra.

2. What is the cause of the diagonal relationship? Also, give examples of diagonal relationships.

Diagonal relationships occur due to the polarisation forces of the diagonally located elements. As it moves along the period, the charge of the ions increases, which increases the polarisation force and decreases the ion size. As you move the group down, the ion size decreases and the polarisation force decreases. If you move diagonally, these effects will be partially offset. The diagonal pairs are Group IA Lithium (Li) and Group IIA Magnesium (Mg), Group IIA Beryllium (Be) and Group IIIA Aluminium (Al), and Group IIIA and Boron (B), and Group IIA Silicon (Si).