Atomic Structure

The maximum number of electrons in each shell,
going from the middle to the outside, is 2, 8, 8, 18.
Below is a picture of a potassium atom. Its electron structure is 2, 8, 8, 1.




Potassium Atom showing Electrons in their Shells

An atom which has the maximum number of electrons in its outer shell
will be stable. This means that it will not react with other atoms.
It belongs to a group called the noble gases. The electron shells
are said to be full. The inner shells of an atom are always full.

If the outer shell of an atom has less than its maximum number of electrons
(see potassium above) then it will not be stable.
It will react with other atoms to get a full outer shell.