The first element, hydrogen, has only one electron orbiting in the first of the electron shells.
As the number of electrons rises, we will look at how the electrons are arranged in the first three shells.
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First electron shell
The first electron shell is the one closest to the nucleus. It is always the first shell to be filled.
It can hold a maximum of two electrons. So, only hydrogen, with one electron, and helium, with its two electrons, can contain all their electrons within this shell.
Second electron shell
Lithium is the next element in the periodic table after helium. It has three electrons.
The first two of these fill the first electron shell. Because that shell is now full, the third electron must orbit in the second electron shell. This second shell can hold a maximum of eight electrons.
Third electron shell
Aluminium has 13 electrons. The first two of these fill the first electron shell, and the next eight fill the second electron shell.
This means that the last three must orbit in the third electron shell. Like the second shell, the third shell can hold a maximum of eight electrons.
Summary of shells
The first electron shell can hold a maximum of two electrons
The second electron shell can hold a maximum of eight electrons
The third electron shell can hold a maximum of eight electrons
