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# Electron Configuration

Humans by nature tend to be systematic and that is achieved by proper arrangement. Humans have tried to arrange the elements too. Efforts were made by various chemists, and Mendeleev was one of them. He arranged the elements by atomic weight.

Electronic configuration is the arrangement of electrons in an atom in different shells and sub-shells. After the discovery of the shells and sub-shells, various physical and chemical properties of the elements were linked to the electronic configuration. As a result, attempts were made to arrange elements by their electronic configuration, so that their properties would obviously fall in a trend. Subsequently, the modern arrangement is evolved wherein elements are organized by atomic number. This is known as the modern periodic table.

The modern periodic table has elements arranged according to their electronic configurations. From left to right, i.e., in a period, the arrangement is such that each subsequent electron is added to the same shell. There are certain steps for writing the electronic configuration.

Step-1

Find out the atomic number of the element from periodic table. Since, in an atom, the number of protons which is the atomic number is equal to the number of electrons, the atomic number will give you the information about the number of electrons in that atom . Let's take the example of calcium. The atomic number of calcium is 20.

Step-2

Now, fill electrons by following the arrows of the electron configuration chart. Remember that the s-orbital can accommodate a maximum of two electrons while the p-orbital sets can hold six electrons in three orbitals. In the same way d-orbital set contains ten and f-orbital sets contain fourteen electrons in five and seven orbitals respectively.

Step-3

After filling given electrons in the energy levels, write the electronic configuration in the given manner.

[X] [Y] [Z]

Where, X represents the energy level, Y is for the sub-shell which is named as s, p, d and f. The number of sub-shells increases with the energy level. The number Z is the number of electrons present in a sub-shell.

We can represent electron configuration chart by using principle quantum number notifications also.

For filling given electrons, we have to start form the K shell and move to higher levels.

## Electronic Configuration in the Period

### Example

Configuration of Li atomic number. 3, is 1s2 , 2s1

Configuration of Be atomic no. 4, is 1s2 , 2s2

Configuration of B atomic no. 5, is 1s2 , 2s2, 2p1

Configuration of C atomic no. 6, is 1s2 , 2s2 , 2p2

and so on till Neon,the last element.

This addition of each subsequent electron results in a completely new configuration and as a result the chemical properties of the subsequent element changes gradually. At the end of each row, a drastic shift occurs in the chemical properties.

Use the below widget to find the electron configuration.

## Electronic Configuration in the Group

From top to bottom in a column, i.e., in a group, each subsequent electron is added to the new shell, but in the same sub-shell, e.g., in the 1st group,

Configuration of H atomic number.1,is 1s1

Configuration of Li atomic no. 3, is 1s2 ,2s1

Configuration of Na atomic no. 11, is 1s2 ,s2,2p6 ,3s1

Configuration of K atomic no. 19, is 1s2 ,2s2 ,2p6,3s2 ,3p6,4s1

and so on till Neon, the last element.

The number of valence shell electrons in an atom would determine the way it will interact with other atoms and therefore determines its chemical properties.

Thus the outermost configuration, which is responsible for various properties such as valency, oxidation states, etc., remains same. Hence in a group, the physical and chemical properties are same. To illustrate, the halogens are strong electronegative elements.

## Valence Electron Configuration

The electrons present in the outer most shell of an atom which takes part in chemical reactions, are known as valence electrons.
These electrons can combine with valence electrons of other atoms to form chemical bonds. Hence valence electrons affect the chemical properties and bonding nature of an atom.

The electronic configuration of the outer most shell of an element is known as valence electron configuration.

It’s easy to write the valence electron configuration for the main group of elements (group 1,2 & 13-18), because the electron present in the electronic shell of the highest principal quantum number are valence electrons and their electronic representation will be easy. For example, the electronic configuration of Cesium (Cs) is 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 5s2 5p6 6s1 so that there is one valence electron in 6s and the valence electron configuration will be 6s1.

But in case of d and f block elements, the valence electron configuration involves the penultimate shell and the ultimate shell also. Hence, the general valence electron configuration of the various blocks is

 Element Valence Electron Configuration s-block element ns1 p-block element ns2 , np 1-6 d-block element (n - 1)d 1-10,ns 1-2 f-block element (n - 2)f 1-14(n - 1)d 0-1nsa

## Electron Configuration Table

We can show the electron configuration of an element in a tabular form also. Such type of tabular form is termed as electron configuration table. This table describes the number of electrons in each energy level of an atom as well as their arrangement. Electron configuration table is the best way to represent an atom as it shows all the information regarding the energy levels of an atom in a small space.

For example, the electron configuration table for Thallium (Tl) with atom number 81 is given below

 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 4f14 5s2 5p6 5d10 6s2 6p1