Electronic Configuration

Valence Shell

The shell which is the furthest away from the nucleus which is occupied by electrons is called the valence shell. The electrons in the valence shell are known as valence electrons. In a chemical reaction, only the valence electrons are involved in chemical bonding between atoms.

Electronic Structure
Electrons occupy shells at different distances from the nucleus. Each shell, however, can only contain a certain number of electrons. This will be explained further in the part on quantum theory.

1st shell → 2 electrons
2nd shell → 8 electrons
3rd shell → 8 electrons for the first 20 elements; 18 electrons for the rest of the elements
4th shell → 32 electrons

A full electronic structure diagram shows the nucleus and every shell of the atom.

Electronic structure of Argon.

Electronic Configuration

A simple way of writing the electronic configuration of an atom is by listing down the number of electrons in every shell, starting from the most inner shell to the most outer shell, using a full stop to separate every shell.
For example, the electronic configuration of argon is 2.8.8.