Octet Rule - Online Tutor, Practice Problems & Exam Prep

The octet rule is a fundamental concept in chemistry where main group elements tend to achieve a stable configuration by having eight electrons in their valence shell, akin to noble gases. This stability is usually achieved through chemical bonding, where covalent bonds allow the sharing of valence electrons between atoms. However, there are exceptions to this rule. Elements with an incomplete octet, such as hydrogen, helium, beryllium, and certain Group 3A elements, are stable with fewer than eight electrons. Conversely, elements capable of an expanded octet—found in period 3 and below, like phosphorus and sulfur—can be stable with more than eight valence electrons due to their larger electron shells. These deviations from the octet rule are crucial for understanding the chemical behavior and bonding capabilities of different elements.

Octet Rule

When you hear the term octet rule, just realize it's the tendency of most main group elements and achieving 8 octet electrons by way of chemical bonding. Now we're going to say they do so to us to have the same number of octet electrons as a noble gas and realize that each covalent bond formed between two elements equals the sharing of two valence electrons between them.

Now we talked about octane electrons. Now valence electrons are electrons in element possesses based on group number. So aluminum being in Group 3A means it has three valence electrons. Now shared electrons are the electrons in element gains through a chemical bond. And when we talk about octet electrons, octet electrons equals these valence electrons plus your shared electrons in.

By combining the valence in shared, this justifies the need to get to 8 total octane electrons for a majority of the main group elements. Remember, hydrogen doesn't fit this definition neatly. It only needs one valence electron to get the noble gas configuration of helium. So we don't talk about hydrogen when it comes to the octet rule.

Now that we've learned about these three different classes of electrons, click on to the next video and let's take a look at an example question.

Octet Rule Example 1

In this example question, it says which of the following statements is true in terms of the following compound. In the following compound we have a nitrogen atom in the center and it is surrounded by three fluorines. Realize here that nitrogen it's a main group element and the drive of main group elements is trying to get to 8 octet electrons. So that means that the answer could be B or C.

Now we can look at its valence electrons. Its valence electrons are the electrons tied to its group number. We're going to say nitrogen's in Group 5A, so it should have 5 valence electrons. These are the electrons that it brings to the table in order to contribute to this chemical compound. So nitrogen's in Group 5A. So this is 12345 electrons it's brought in.

The shared electrons are the electrons that it gains from chemical bonding to fluorine atoms. So we'd say here that it's shared electrons. If we take a look it is gaining 123 additional electrons from the fluorine atoms and that 5 + 3 is what gives us 8 octet electrons. So here the best answer would be option B.

So just keep in mind elements come in with their electrons and in order to get to the 8 octet electrons, they can share electrons from the surrounding elements around them.

How many shared electrons are around the oxygen atom?

• A. 4
• B. 2
• C. 6
• D. 10
• E. 8

Which of the following atoms has the most valence electrons?

• A. O
• B. Be
• C. He
• D. N
• E. Si

Octet Rule

Now how could it be chemistry without some exceptions? We learned that a majority of the main group elements want to have 8 octet electrons around themselves to resemble more like the noble gases. But now we're going to see that some elements can have a non octet number of electrons and be completely fine and stable. So we're going to say these elements are stable with a non octet number of electrons.

We talk about incomplete octets and expanded octets. In an incomplete octet, an element is able to maintain its stability when having less than 8 octet electrons around itself, and in an expanded octet, another element is able to maintain its ability when it has more than 8 octet electrons around itself. Now we're going to say here that their non octet number of electrons typically is twice their group number.

So if we take a look if we're looking at a group two way element twice. So 2×2 will mean that it's OK with having 4 electrons for its number of octet electrons. Here group 3A is OK with having 6 octet electrons, 10 octet electrons for six on 5A, 12 octet electrons from element in Group 6A and then we have 14 and then 16.

If we take a look here, the elements that typically have an incomplete OP tech are hydrogen, helium, beryllium and then in Group 3 as boron, aluminum, gallium and Indium. The elements that typically can have an expanded octet, they are found in row three or. Three and lower. So we're talking about phosphorus, sulfur, chlorine, arsenic, selenium, bromine, Krypton, tellurium, iodine, xenon and radon.

Now remember, noble gases are very stable. They tend not to want to get involved in bonding preferences because they're perfect, but these ones that are lowered down, since they're so much lower, they have larger shells so they can form bonds if needed. And Justice realize here that when it comes to their non op tech number they're not going to have 8 octet electrons. They tend to have this twice the value of their group number. So keep that in mind when examining these different types of elements that have non octet number of electrons.

Octet Rule Example 2

Here it says how many octet electrons are around the phosphorus atom in the following compound. So remember your octet electrons equals your valence electrons plus your shared electrons. And if we take a look here, your valence electron number is based on your group number. Phosphorus is in Group 5A so it has five valence electrons, so 1, 2, 3, 4, 5.

And then the shared electrons are the electrons you gain by forming a chemical bond with another element. So here our shared electrons would be the ones that we're gaining from forming a bond with the hydrogen atoms SO 1, 2, 3, 4, and 5. So here we'd see that phosphorus has 10 total octet electrons.

Here this makes sense because we said that when we're dealing with a non octet number of electrons, it's typically twice the group number of the element. Phosphorus is in Group 5A, and if you multiply that by two we typically can see it go all the way up to 10 valence electrons. So that means option B would be our correct choice.

Which of the following contains an atom that may have an incomplete octet?

• A. CCl₄
• B. NCl₃
• C. OCl₂
• D. SeCl₆
• E. BCl₃

Here’s what students ask on this topic:

What is the octet rule?

The octet rule is a chemical rule of thumb that reflects a pattern observed in the outermost electrons of atoms in stable molecules. It states that atoms tend to combine in such a way that they each have eight electrons in their valence shell, giving them the same electronic configuration as a noble gas. The rule is based on the principle that having a full or complete outer shell with eight electrons is energetically favorable and confers stability.

This rule helps explain why atoms form certain types of ions and molecules. For instance, sodium ($\mathrm{Na}$) has one electron in its outer shell and will typically lose that electron to achieve an octet in the next lower shell, becoming a positively charged ion (${\mathrm{Na}}^{+}$). Chlorine ($\mathrm{Cl}$), on the other hand, has seven electrons in its valence shell and will gain an electron to complete its octet, becoming a negatively charged ion (${\mathrm{Cl}}^{-}$). When these ions combine to form sodium chloride ($\mathrm{NaCl}$), both achieve stable octet configurations. However, it's important to note that the octet rule has exceptions and doesn't apply to all elements, particularly those beyond the second period of the periodic table.

What are the exceptions to the octet rule?

The octet rule states that atoms tend to combine in such a way that they each have eight electrons in their valence shell, giving them the same electronic configuration as a noble gas. However, there are several exceptions to this rule:

1. Incomplete Octet: Some elements are stable with less than eight electrons in their valence shell. Examples include hydrogen (which needs only two electrons to fill its shell), boron, and aluminum which often form stable compounds with six valence electrons.
2. Odd-Electron Molecules: Molecules with an odd number of electrons, like nitric oxide (NO), cannot distribute electrons to give eight to each atom, resulting in an unpaired electron.
3. Expanded Octet: Elements in the third period and beyond can have more than eight electrons because they have d orbitals available for bonding. Examples include phosphorus (which can have 10 electrons in compounds like PF₅) and sulfur (which can have 12 electrons in compounds like SF₆).
4. Ionic Compounds: In ionic compounds, atoms do not share electrons to achieve noble gas configuration; instead, they transfer electrons. As a result, the octet rule does not apply in the same way as it does to covalent compounds.

Which elements do not follow the octet rule?

The octet rule is a chemical guideline that reflects the observation that atoms of main-group elements tend to bond in such a way that each atom has eight electrons in its valence shell, giving it the same electronic configuration as a noble gas. However, there are several exceptions to this rule:

• Hydrogen and Helium: These elements follow the "duet rule," as they are stable with just two electrons in their outermost shell.
• Boron and Beryllium: These elements can be stable with six and four electrons, respectively, in their valence shell.
• Elements in Period 3 and beyond: These elements can have more than eight electrons due to the availability of d-orbitals. Examples include phosphorus (which can have 10 electrons), sulfur (12 electrons), and chlorine (14 electrons).
• Molecules with an odd number of electrons: For example, nitric oxide (NO) and nitrogen dioxide (NO₂) do not have an even number of electrons to distribute into pairs for an octet.
• Ions or molecules with less than an octet: Some compounds are stable with fewer than eight electrons around an atom, such as the aluminum ion in AlCl₃.

These exceptions occur because the elements either do not have enough electrons to complete an octet or have the capacity to expand their valence.