An electronic configuration is the way in which electrons are arranged in an atom.
Electrons in shells
Different shells can hold different maximum numbers of electrons. Electrons occupy shells starting with the innermost one. They begin to occupy the next shell when a shell becomes full.
For elements with atomic number 1 to 20:
| Electron shell | Max electrons |
| First | 2 |
| Second | 8 |
| Third | 8 |
Predicting an electronic configuration
The electronic configuration of an atom can be predicted from its atomic number. For example, the atomic number of sodium is 11. Sodium atoms have 11 protons and so 11 electrons:
- 2 electrons occupy the first shell
- 8 electrons occupy the second shell
- 1 electron occupies the third shell
This electronic configuration can be written as 2.8.1 (each dot separates one shell from the next). This electronic configuration can also be shown as a diagram. In these diagrams:
- each shell is modelled as a circle
- each electron is modelled as a dot or a cross
Electronic configurations and the periodic table
The electronic configuration of an element is related to its position on the periodic table.
The links are:
- the number of circles in the electronic configuration of an element is represented in the periodic table as the period number that element is situated in
- the number of electrons in the outermost shell of an element is represented in the periodic table as the group number that element is situated in
- the number of electrons in all shells of an element is represented in the periodic table as the element’s atomic number
| Electronic configuration feature | Link to the periodic table |
| Number or numbers of circles | Period number |
| Number of electrons in outermost shell | Group number |
| Total number of electrons in all shells | Atomic number |
The electronic configuration of sodium (2.8.1) shows that sodium, Na:
- is in period 3
- is in group 1
- has an atomic number of (2 + 8 + 1) = 11
Electronic configurations and properties of elements
The electronic configurations of atoms help explain the properties of elements and the structure of the periodic table. When atoms collide and react, it is the outer electrons that meet and interact. So, elements in the same group have similar chemical properties because they have the same number of electrons in their outer shell.
The electronic configurations of the elements in group 1:
The atoms of all group 1 elements have similar chemical properties and reactions because they all have one electron in their outer shell. Similarly, the atoms of all group 7 elements have similar chemical properties and reactions to each other because all of them have seven electrons in their outer shell.
