The Octet Rule or Octet Theory states that atoms must have eight electrons in their valence shell in order to acquire chemical stability.
The octet rule says:
"in a chemical bond, an atom tends to have eight electrons in its valence shell in the ground state, similar to a noble gas"
In order for atoms to present the complete valence layer, chemical bonds must be made in order to donate, receive or share electrons.
Atoms tend to share electrons until they acquire a stable configuration, that is, the complete valence shell.
In this way, an atom presents the eletronic distribution equal to a noble gas closest to its atomic number.
The noble gases (Family 8A) are elements in the periodic table that have eight electrons in the valence shell. The only exception is the element Helium which has 2 electrons.
When the atom has eight electrons in the valence shell, it achieves its stability. This means that it will not bind to other atoms, as it has no tendency to gain or lose electrons.
That's why we didn't find compounds formed by noble gases.
Examples
Check out some examples of chemical bonds made in order to reach the eight electrons in the valence layer:
chlorine
O chlorine (Cl) has atomic number 17 and seven electrons in the valence shell. So for it to become stable it needs an electron.
Therefore, it needs to share a pair of electrons through chemical bonds. One way is to bond with another chlorine atom and form the Cl molecule.2.
Thus, the eight electrons in the valence shell are reached, satisfying the octet rule.
Oxygen
Oxygen has six electrons in the valence shell. To become stable, it needs two more electrons that will be obtained through chemical bonds.
Oxygen can bond with two hydrogen atoms and form a molecule of water. This is a covalent bond and each hydrogen shares one of its electrons.
Thus, oxygen has eight electrons in the valence shell.
To learn more about chemical bonds, read also:
- Chemical bonds
- Ionic Bonds
- Covalent bond
Exceptions
As with every rule, there are exceptions. The exceptions to the octet rule cover elements that do not need eight electrons in the valence shell to be stable.
Check out some cases of exceptions to the octet rule:
Stable elements with less than eight electrons
Also called contraction of the octet, it is more common to happen with elements from the second period of the periodic table.
Included in this exception are elements that with less than eight electrons in the valence shell already become stable.
An example is the element Beryllium (Be), it becomes stable with only four electrons in the last shell.
Boron (B) and Aluminum (Al) become stable with six electrons in the valence shell.
Stable elements with more than eight electrons
Also called octet expansion, happens with non-metallic elements from the third period. Because they have more electron shells, they also have more orbitals available to receive electrons.
This situation occurs with Phosphorus (P) and Sulfur (S). Phosphorus can receive up to 10 electrons and sulfur 12 electrons.
Read too:
- electronegativity
- electropositivity
- Periodic table
