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 shell||Maximum number of electrons|
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.
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.