Electronic structure

Electron shells

Electrons are arranged in different shells around the nucleus. Each successive shell can only hold a certain number of electrons.

The innermost shell is filled first. This shell can contain a maximum of two electrons. The second shell can hold a maximum of eight electrons. When this is filled, electrons go into the third shell, which also holds a maximum of eight electrons. Then the fourth shell begins to fill.

Energy shellMaximum number of electrons
First2
Second8
Third8

A lithium atom, for example, has three electrons. It has two in the first shell and one in the second shell.

Lithium atom with three electrons orbiting the nucleus: two in the first energy shell, one in the second.Electronic arrangement of lithium

A carbon atom has six electrons. It has two in the first shell and four in the second shell.

Carbon atom with six electrons orbiting the nucleus: two in the first energy shell, four in the second.Electronic arrangement of carbon

A calcium atom has 20 electrons. Two are in the first shell, eight in the second shell, eight in the third shell, and two in the fourth shell.

Calcium atom with 20 electrons orbiting the nucleus: two in the first energy shell, eight in the second shell, eight in the third shell and two in the fourth shell.Electronic arrangement of calcium
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