In this tutorial, you will learn what the octet rule is, the exceptions to the rule, and see examples and problems.
What is the Octet Rule?
The octet rule is that an atom will be most stable when surrounded by 8 electrons in the valence shell. An atom that does not have eight electrons will bond with other atoms to have eight electrons. A configuration that has eight electrons is also referred to as the ‘noble-gas configuration’.
The noble gasses are the only atoms with eight valence electrons and therefore tend not to react or bond with other atoms and molecules. This lack of reaction is because they are already at their most stable state with a full octet of electrons. Hence why other atoms with a full octet are referred to as in ‘noble-gas configuration’.
The filled valence octet means that the s and p-orbitals are filled. Therefore, the electron configuration ends with s2p6 for all atoms with a full octet.
Achieving the Octet Rule
Generally, metals will lose electrons to achieve a valence shell of eight electrons. Non-metals tend to gain electrons to obtain eight electrons.
Certain atoms tend to have a certain number of bonds to meet the octet rule. For example, carbon will almost always have four bonds. Nitrogen will almost always have three bonds plus a lone pair. Oxygen normally has two bonds. Fluorine normally has one. You should notice a trend. As you move across the row of the periodic table, the number of bonds tends to decrease for those elements (non-metals) that are covalently bonding.
This trend of decreasing the number of bonds holds true for the rows below the one discussed above. The further down you move in the periodic table the more exceptions to the octet rule there are.
Exceptions to the Octet Rule
- Transition metals do not normally follow the octet rule. Instead, because of the d-block electrons, they can have more electrons in the valence shell.
- Helium and hydrogen both do not follow the octet rule, but instead have two electrons in the valence shell to be stable. This is because they do not have p-orbitals, and just need the s2 orbital filled.
- Lithium often will lose an electron to have the same configuration as helium (2 electrons).
- Some compounds disobey the octet rule. For example, SF6 and XeF2 are two commonly made gasses in labs. The further down an atom is in the periodic table, the more likely it is to not have eight electrons in the valence shell and still be stable.
Here are several example problems explaining how to count electrons to determine if an atom is following the octet rule. The molecules are drawn using Lewis dot structures.
N2 – Nitrogen
There are three covalent bonds between the nitrogen atoms and each nitrogen has a lone pair. Therefore, the total number of electrons associated with each nitrogen atom is 8 and both atoms follow the rule.
CH4 – Methane
In methane (CH4) there are two different atoms to look at. We will start with carbon which has four covalent bonds. There are two electrons associated with each covalent bond, therefore carbon is following the octet rule.
The hydrogen in CH4 all have one covalent bond associated with them, giving them two total electrons. Remember that hydrogen is an exception to the rule though and stable with two electrons!
Problem: Using what you know about the octet rule, draw the structure for HCN
Solution: Both carbon and nitrogen will follow the octet rule and hydrogen will have two electrons. We have a total of 10 electrons to work with. A good place to start is giving nitrogen three bonds to carbon. The carbon then needs one more bond, which fits with the one bond hydrogen needs. Structure solved!
Problem: Using what you know about the octet rule, draw the structure for CH2O
Solution: To start, we will make the informed prediction that carbon will have four bonds, oxygen will have two, and hydrogen will have one. Counting all the valence electrons, we have 12 electrons to work with. Most likely, carbon will be the central atom because it needs the most bonds. From there, we can connect it to oxygen with a double bond. Oxygen now needs two lone pairs to satisfy the octet rule. Carbon needs two more bonds, one to each hydrogen. We can count all the electrons in our structure and see that we have 12 electrons, and every atom satisfies the octet rule (with hydrogen’s exception).