1.1.10 Electronic Structure

Energy Levels:

• Electrons in an atom occupy energy levels. These are also called shells or orbits. Each electron in an atom is found in a particular energy level.
• The lowest energy level (innermost shells) fills with electrons first.
• Each energy level can only hold a certain number of electrons before it becomes full.
• The first energy level can hold a maximum of two electrons, the second energy level a maximum of eight, and so on.

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Electrons in the first three energy levels for elements with atomic numbers 1 to 20 are distributed as follows:

Energy level or shell	Maximum number of electrons
First	2
Second	8
Third	8

Writing an Electronic Structure

The electronic structure of an atom is written using numbers to represent the electrons in each energy level.

For example, for sodium, this is 2,8,1 – showing that there are:

• 2 electrons in the first energy level.
• 8 electrons in the second energy level.
• 1 electron in the third energy level.

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