In a football game, a team can’t have more than 11 players on the field at a time. But having fewer players can also be a disadvantage. Atoms are similar in that they prefer to have a certain number of electrons in their outer shell, typically eight. This is called the octet rule. Basically, atoms are most stable when they have eight electrons in their outer shell. This rule helps us predict how atoms will bond with each other in chemistry.
Now that we understand the octet rule, let’s explore how it works with Lewis diagrams. We’ll also look at some examples to see the octet rule in action. However, it’s important to note that there are some exceptions to the rule. So, while it’s a helpful tool in chemistry, it’s not a hard and fast rule.
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What is the octet rule?
Noble gases are also called inert gases, which means they don’t react with other elements easily. They are odourless, colourless, and have high ionization energies and low electron affinities. Their electron configurations explain why they are so unreactive – they all have full outer shells of electrons, which makes them very stable. The outer shell of electrons is called the valence shell, and having a full valence shell is desirable because it makes an atom extremely stable. This is known as the octet rule, which states that atoms are most stable when they have eight outer shell electrons, like noble gases. Other atoms try to gain or lose electrons until they have eight outer shell electrons to become stable. Although it was believed that noble gases didn’t react with any other element, we now know that they do react under certain conditions.
Obeying the octet rule
Thanks to the octet rule, it is easy to predict how certain elements react when they bond. For example:
Group I metals have one electron in their outer shell. They tend to lose this electron in order to achieve a noble gas electron configuration. Group II metals have two electrons in their outer shell. They tend to lose these two electrons in order to achieve a noble gas electron configuration. Group VI non-metals have six electrons in their outer shell. They tend to gain two electrons in order to achieve a noble gas electron configuration. Group VII halogens have seven electrons in their outer shell. They tend to gain one electron in order to achieve a noble gas electron configuration.
The octet rule and Lewis diagrams
The octet rule is very useful in predicting the structure of a molecule. It helps us draw Lewis diagrams, which show the arrangement of atoms and electrons in a molecule. To draw a Lewis diagram, we need to follow a few steps. First, we count the total number of valence electrons in the molecule. Next, we draw the rough position of the atoms and join them using single covalent bonds. Then, we add electrons to the outer atoms until they have full outer shells. After that, we count how many electrons we have left and add them to the central atom. Finally, we use double covalent bonds to form complete outer shells on all the atoms.
Let’s take carbon dioxide (CO2) as an example. Carbon has four valence electrons, and each oxygen has six valence electrons, so the total number of valence electrons in CO2 is 16. We draw a central carbon atom with two outer oxygen atoms joined to it using single covalent bonds. We then add electrons to the outer oxygen atoms until they have full outer shells. Finally, we add the remaining electrons to the central carbon atom, using double covalent bonds to form complete outer shells on all the atoms.
Next, we use our first application of the octet rule. Each atom wants to have eight valence electrons, giving it a full outer shell. We start by looking at the outer atoms. In this case, these are the two oxygen atoms. Both currently only have two valence electrons from the covalent bond that they share with carbon. To get to a full outer shell, each oxygen needs to gain six more electrons. Let’s draw them in.
If we add up the total number of valence electrons we’ve added to the molecule, we find 2(2) = 4 electrons from the two single bonds, and 6(2) = 12 electrons from the lone pairs. 12 + 4 = 16, which you might remember is the number of valence electrons that carbon dioxide is allowed to have. We can’t add anymore electrons. However, our Lewis structure isn’t complete. This is where the octet rule comes in again. The central carbon atom currently only has four electrons in its outer shell; to achieve a full outer shell and satisfy the octet rule, it needs eight. To give it four extra electrons, we use a lone pair of electrons from each oxygen atom to form two C=O double bonds.
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All atoms now satisfy the octet rule. Our structure is complete.
Octet rule limitations and exceptions
Although the octet rule is a great model, it doesn’t always hold up. In fact, it has some notable exceptions.
Odd number of electrons
Some molecules have odd numbers of electrons. Because of this, it is impossible for them to obey the octet rule. These include free radicals such as nitric oxide and nitrogen dioxide.
Incomplete octets
While most atoms follow the octet rule, there are some exceptions. Hydrogen and lithium, both members of period 1, are stable with just two valence electrons. When they bond, they take the electron configuration of helium, which has a full outer shell with just an s-subshell. This means they only have space for a single electron pair, so having just two valence electrons satisfies their desire to have a full outer shell.
Boron and aluminium, members of group III, are also able to form stable molecules with incomplete octets. For example, boron trifluoride (BF3) consists of a central boron atom joined to three outer fluorine atoms by single covalent bonds. The fluorine atoms have complete octets, but boron only has six electrons in its outer shell. However, boron finds this arrangement stable and does not react any further. Aluminium trichloride acts in much the same way. These exceptions to the octet rule are important to keep in mind when predicting the structure and reactivity of molecules.
Expanded octets
Some atoms, specifically those in period three and beyond, are able to form expanded octets. This means that the atom has more than eight electrons in its outer shell. These electrons go in the d-subshell, which is why elements in periods 1 and 2 can’t form expanded octets – they don’t have a d-subshell. This is also why some noble gases can form bonds. As we’ve explored, noble gases are generally unreactive because they already have a full outer shell of electrons. However, noble gases in period 3 and beyond can form bonds with other atoms; the extra bonded electrons go in the d-subshell.
One example of a molecule with an expanded octet is phosphorus pentachloride. This molecule consists of a central phosphorus atom bonded to five chlorine atoms using single covalent bonds. Phosphorus has ten electrons in its valence shell, giving it an expanded octet.
Another example is xenon tetrafluoride, consisting of a central xenon atom joined to four fluorine atoms with single covalent bonds. It has twelve electrons in its outer shell.
In summary, the octet rule is a general rule in chemistry used to predict the bonding between atoms. It states that atoms are most stable when they have eight electrons in their outer shell, giving them the electron configuration of a noble gas. We can use the octet rule to draw Lewis diagrams, making sure that each atom has eight electrons in its outer shell. However, there are exceptions to the octet rule, such as molecules with odd numbers of electrons and some atoms that can form incomplete or expanded octets. Understanding these exceptions is important for predicting the structure and reactivity of molecules.
The Octet Rule
What does the octet rule state?
The octet rule is a general rule in chemistry used to predict the bonding between atoms. It states that atoms are at their most stable when they have eight electrons in their outer shell, giving them the electron configuration of a noble gas.
What is the octet rule with an example?
The octet rule is a general rule in chemistry used to predict the bonding between atoms. It states that atoms are at their most stable when they have eight electrons in their outer shell, giving them the electron configuration of a noble gas. An example is the oxygen molecule, O2. The two oxygen atoms join using a covalent double bond so that both atoms have eight electrons in their outer shell.
What is the octet rule and why is it important?
The octet rule is a general rule in chemistry used to predict the bonding between atoms. It states that atoms are at their most stable when they have eight electrons in their outer shell, giving them the electron configuration of a noble gas. It is important because it helps us predict the structure and bonding within a molecule.
How does calcium obey the octet rule?
Calcium bonds by losing its two outermost electrons. This gives it a full outer shell with eight valence electrons, obeying the octet rule.
Which elements do not follow the octet rule?
Hydrogen and helium are among the elements that don’t follow the octet rule. They only aim to have two electrons in their outer shell. Other elements that ignore the octet rule are aluminium and boron. They can form molecules in which there are only six electrons in their outer shell.
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