What is the octet rule of reactivity?

A football team can only send 11 players onto the pitch at any one time. They're not allowed anymore players - if they go over 11, they have to send someone off. But too few players is also a disadvantage, and in that case, the manager will try to send someone else on. You could say that football teams are at their best with exactly eleven players. Atoms are quite similar. Generally, they prefer to have a certain number of electrons in their outer shell - no more, no less. This number is typically eight. We call this the octet rule.

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.

• This article is about the octet rule in chemistry.
• We'll define the octet rule before looking at the octet rule and Lewis diagrams.
• After that, we'll explore some examples of the octet rule.
• We'll then learn about the limitations of the octet rule, including notable exceptions.

What is the octet rule?

Noble gases are historically known as inert gases. Their name gives us a clue about their behavior. They are odorless, colorless, have high ionization energies and low electron affinities, and are generally extremely unreactive. In fact, it was long believed that they didn't react with any other element at all (although we now know that this is incorrect, as you'll see later on in the article).

If we take a look at their electron configurations, we discover an explanation for the relative inertness of noble gases: they all have full outer shells of electrons. For all of the noble gases apart from helium, this means having eight electrons in their outer shell.

An atom's outer shell is known as its valence shell, and its outer shell electrons are known as valence electrons.

Having a full outer shell of electrons makes an atom extremely stable, which is highly desirable. Therefore, other atoms tend to try to gain or lose electrons until they have full outer shells, and because this usually involves having exactly eight valence electrons, this phenomenon is known as the octet rule.

The octet rule states that atoms are at their most stable when they have the electron configuration of a noble gas. This means having eight outer shell electrons. Because of this, atoms tend to try and gain or lose electrons until they have eight electrons in their outer shell.

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

We can use the octet rule to help us predict the likely structure of a molecule when faced with multiple different Lewis diagrams. It is also helpful when drawing Lewis diagrams from scratch.

We have a whole article dedicated to drawing Lewis diagrams over at "Lewis dot diagrams". Head over there for a more detailed explanation.

Here's how you go about it:

1. Work out the molecule's total number of valence electrons.
2. Draw the rough position of the atoms in the molecule.
3. Join the atoms using single covalent bonds.
4. Add electrons to the outer atoms until they have full outer shells of electrons.
5. Count up how many electrons you have added, and subtract this from the molecule's total number of valence electrons that you calculated earlier. This tells you how many electrons you have left.
6. Add the remaining electrons to the central atom.
7. Use lone pairs of electrons from the outer atoms to form double covalent bonds with the central atom until all atoms have complete outer shells.

We'll start by applying this process to carbon dioxide, CO2. First of all, we need to count up the molecule's number of valence electrons. Carbon is in group IV and so has four valence electrons. Oxygen is in group VI and so has six valence electrons. The molecule's total number of valence electrons is therefore 4 + 2(6) = 16.

We then draw the rough position of the atoms in the molecule and add single covalent bonds between them. Here, we draw a central carbon atom joined to two outer oxygen atoms using single covalent bonds.

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.

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

Other atoms, particularly the two members of period 1 and the smaller members of group III, are able to form molecules with incomplete octets. Hydrogen and lithium are both stable with just two valence electrons. When they bond, they take the electron configuration of helium, 1s2. The outer electron shell contains just an s-subshell, meaning it only has space for a single electron pair. Therefore, having just two valence electrons satisfies their desire to have a full outer shell.

Boron and aluminium are also able to form stable molecules with incomplete octets. Take boron trifluoride, BF3. It consists of a central boron atom joined to three outer fluorine atoms by single covalent bonds. The fluorine atoms have complete octets, but boron does not: it only has six electrons in its outer shell. However, boron finds this arrangement stable and so doesn't react any further. Aluminium trichloride acts in much the same way.

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.

A molecule with an expanded octet: xenon tetrafluoride. Image credit: commons.wikimedia.org

That's it for this article. With any luck, you should now understand what we mean by the octet rule and be able to draw Lewis diagrams using the octet rule for guidance. You should also be able to identify exceptions to the octet rule.

The Octet Rule - Key takeaways

• 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.
• We can use the octet rule to help us draw Lewis diagrams. When assigning electrons to atoms, make sure that each atom has eight electrons in its outer shell.
• There are some exceptions to the octet rule:
  • Molecules with odd numbers of electrons disobey the octet rule. These include free radicals such as nitric oxide and nitrogen dioxide.
  • Some atoms can form incomplete octets with fewer than eight valence electrons. These include boron and aluminium. Hydrogen, helium and lithium always form incomplete octets with just two outer shell electrons.
  • Some atoms can form expanded octets with more than eight valence electrons. The extra electrons go in a d-subshell. Molecules with expanded octets include phosphorus pentachloride and xenon tetrafluoride.

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.

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.

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.

Calcium bonds by losing its two outermost electrons. This gives it a full outer shell with eight valence electrons, obeying 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.

Question

Answer

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.

Question

How many valence electrons do most atoms try to achieve, according to the octet rule?

Question

According to the octet rule, atoms try to achieve the electron configuration of _____.

Answer

Question

Which molecules feature an incomplete octet?

Question

Which of the following groups loses an electron in order to achieve an octet?

Answer

Question

Which of the following groups gains an electron in order to achieve an octet?

Answer

Question

Why does hydrogen disobey the octet rule?

Answer

Hydrogen's outermost electron shell has room for just two electrons. This means that it is stable with two valence electrons instead of eight.

Question

Which of the following elements can disobey the octet rule?

Question

Why can't the compound NCl5 exist?

Answer

The central nitrogen atom would have ten valence electrons. However, nitrogen is in period 2 and only has room for eight electrons in its outer shell.

Question

Elements in _____ and beyond can form expanded octets.

Answer

Question

Explain why some atoms can form expanded octets.

Answer

Elements in period 3 and beyond can form expanded octets because the extra valence electrons go into a d-subshell. Elements in periods 1 and 2 can't form an expanded octet because they don't have a d-subshell available. 

Question

Which of the following have a complete octet?

Question

What are the three exceptions to the octet rule?

Answer

1. Odd number of electrons

2. Incomplete octets

3. Expanded octets

Question

Which other elements, apart from hydrogen, always form incomplete octets with just two outer shell electrons?

Answer

Question

Give 2 examples of molecules that have an odd number of electrons.

Answer

1. Nitric oxide

2.Nitrogen dioxide