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Atomic Structure

What are Electron Shells (energy levels)?

Electrons in atoms are in shells
(shown as circles around the nucleus).
The shells can also be called energy levels.
We will use the term shell rather than energy level
but either is acceptable.

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 that 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. When the outer shell
has the maximum number of electrons, 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.

Ions also have a full outer shell of electrons but
because they have a charge they will react and
form ionic bonds with other oppositely charged ions.

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