Electrons occupy different energy levels, or shells. Each shell can hold a maximum number of electrons. Moving through the elements in the periodic table, each atom has one more electron than the last because the number of electrons is the same as the atomic number. Electrons occupy the shells in order, starting with the shell that is nearest the nucleus. They begin to occupy the next shell only when this shell becomes full.
For elements with atomic number 1 to 20:
|Electron shell||Maximum number of electrons|
The electron arrangement of an atom can be worked out from its atomic number. For example, the atomic number of sodium is 11. Sodium atoms have 11 protons and so 11 electrons.
This electron arrangement can be written as 2.8.1 (each dot separates one shell from the next). It can also be shown as a diagram. In these diagrams: