Electronic configuration

1. Electrons occupy shells around the nucleus.

2. The lowest energy levels are filled first, meaning the electrons fill the shells closest to the nucleus before moving to higher ones.

3. Only a certain number of electrons can fit in each shell:

   • 1st shell: maximum of 2 electrons

   • 2nd shell: maximum of 8 electrons

   • 3rd shell: maximum of 8 electrons

4. Atoms are more stable and less reactive when they have full outer shells (like the noble gases in Group 0).

5. In most atoms, the outer shell is not full, which causes them to be reactive as they seek to gain, lose, or share electrons to achieve a full outer shell.

Representing electronic configuration

The electronic structure of an atom can be represented by either a diagram, showing the electrons in each shell, or numbers, which list the number of electrons in each shell.

For example, the electronic structure of sodium (Na) can be written as:

• A diagram: Two electrons in the first shell, eight in the second shell, and one in the third shell.

• Numbers: 2,8,1.

Both representations show the same information about how sodium's electrons are arranged.

A sodium atom

Working Out Electronic Structures:

You can use the rules above to work out the electronic structure of elements in the periodic table for the first 20 elements.

Example 1: Nitrogen (N)

• Nitrogen has an atomic number of 7, meaning it has 7 electrons.

• Using the shell rules, its electronic structure is 2,5 (2 electrons in the 1st shell, 5 in the 2nd shell).

A nitrogen atom

Example 2: Magnesium (Mg)

• Magnesium has an atomic number of 12, so it has 12 electrons.

• The electronic structure is 2,8,2 (2 electrons in the 1st shell, 8 in the 2nd shell, and 2 in the 3rd shell).