3.7 ELECTRONIC CONFIGURATION

To understand electronic configuration, you should know about shells and sub-shells.

Shells

According to Bohr's atomic theory, the electron in an atom revolves around the nucleus in one of the circular paths called shells or orbits. Each shell has a fixed energy. So each shell is also called energy level. Each shell is described by an n value. n can have values 1,2,3.....

  • n = 1 it is K shell
  • n = 2 it is L shell
  • n = 3 it is M shell etc.

As the value of n increases the distance of electron from the nucleus and energy of the shell increases.

Sub-Shells

A shell or energy level is sub divided into sub-shells or sub-energy levels. n value of a shell is placed before the symbol for a sub-shell.

  • n = 1 for K shell. It has only one sub-shell which is represented by 1s.
  • For L shell n = 2. L shell has two sub-shells, these are designated as 2s and 2p.
  • For M shell n = 3. M shell has 3 sub-shells called 3s, 3p and 3d.
  • N shell has 4s, 4p, 4d and 4f sub-shells.
  • s sub-shell can accommodate maximum 2 electrons.
  • p sub-shell can accommodate maximum 6 electrons.
  • d sub-shell can accommodate maximum 10 electrons.
  • f sub-shell can accommodate maximum 14 electrons.

The increasing order of energy of the sub-shells belonging to different shells is given below:

1s < 2s < 2p < 3s < 3p < 4s < 3d .....

The arrangement of electrons in sub-shells is called as the electronic configuration. We can fill the electrons present in various elements by using Auf Bau Principle. According to this principle, electrons fill the lowest energy sub-shell that is available first. This means electron will fill first 1s, then 2s, then 2p and so on.

Symbols for atoms and ions

The symbol for an atom represent the element. It consists of one or two-letters, the mass number as a left superscript, the atomic number as left subscript, and the charge as a right superscript. For example:

²⁴₁₂Mg²⁺

This number is often omitted. This diagram shows symbol for magnesium "Mg" which stands for magnesium. The number to the upper left of the symbol is the mass number, which is 24. The number to the upper right of the symbol is the charge which is positive 2. The number to the lower left of the symbol is the atomic number which is 12.