Electrons are arranged in different shells around the nucleus. Each successive shell can only hold a certain number of electrons or say shell can hold only a specific number of electrons. The general formula to determine the number of electrons is 2(n^2), where n is the shell number. The first shell can have two electrons, the second shell can contain up to 8 electrons and the third shell can hold 18 electrons and so on.
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.
| First | 2 |
| Second | 8 |
| Third | 8 |
A lithium atom, for example, has three electrons. It has two in the first shell and one in the second shell. A carbon atom has six electrons. It has two in the first shell and four in the second shell.
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