How to Write Electronic Configuration

Electrons are much disciplined runners. As you know electrons move around the nucleus in certain orbits. Like any racing game, electrons also have to follow some rules. These rules decide the orbit of an electron, space ship it will travel on, and also the particular seat for it. When you see an atom you will find it much organised, even you can guess the arrangement of electrons in it. But you must know the rules.

Rule 1 is about orbit.

It is named as “The Aufbau Principle” (aufbau is German for “building up”). Orbit nearer to the nucleus, which has lower energy, is to be filled first. It means orbit 1will be filled first then 2 then 3 and so on.

If you take an example of element Lithium (³Li), it has 3 electrons. First they have to fill orbit 1 and then move on to the next one. Orbit 1 allows 2 electrons so 2 of 3 electrons will there and remaining 1 electron will go to the orbit 2.

Rule 2 is about space ship.

Space ship is called “Sub-Shell”. You know every orbit has different sub-shells. Orbit 1 has “s” sub-shell, orbit 2 has “s” and “p” sub-shells, orbit 3 has “s”, “p” and “d” sub-shells and orbit 4 has “s”, “p”, “d” and “f” sub-shells.

Every sub-shell has different energy and its energy also depends on the energy of the orbit it belongs to. Orbital “s” is the lowest energy sub-shell and “f” is the highest energy sub-shell.

Energy order of the sub-shells is as follows: s < p < d < f

When you compare the energy of sub-shells belonging to different orbits you will find

1s < 2s < 2p < 3s < 3p < 3d

Let’s take an example of Boron (⁵B), it has 5 electrons. First 2 electrons will go in 1s, next 2 electrons will go in 2s and remaining 1 will go in 2p. Now you are able to distribute electrons in orbits and sub-shell. How do you write your finding in a precise manner? 

·               ​First write orbit number before sub-shell,

·               Number of electrons is to be written in the right superscript.

This way of presentation of electrons distribution is called “Electronic Configuration”. Now try to write electronic configuration of Boron​.⁵ B = 1s², 2s², 2p¹

Rule 3 is about assigning the seats in a spaceship.

You know every spaceship or sub-shell has different number of compartments. These compartments are named “Orbitals”. Orbitals of the same sub-shell have equal energy and these are called “Degenerate Orbitals”. Every orbital can accommodate maximum 2electrons.

This rule is called “Hund’s Rule” (given by the scientist Friedrich Hermann Hund). It states that when there are degenerate orbitals electron will occupy an empty orbital before it will pair up with another electron.

Let’s take an example of Oxygen (⁸O), it has 8 electrons. And its electronic configuration will be ⁸O = 1s², 2s², 2p⁴

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If you remember I have told you that electrons are like spin top. Every electron revolves clockwise or anticlockwise in its axis while it travels in the orbit. When they share single orbital one of the electron revolves clockwise and the other one anticlockwise. Clockwise rotation is represented by half up side arrow and anticlockwise rotation by half down side arrow. Let’s place electrons of oxygen in their corresponding orbitals.

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