Lewis Dot Structures: Exceptions: Videos & Practice Problems

In chemistry, the octet rule is a guideline that elements tend to combine 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 exceptions to this rule. Elements in Group 2A can be stable with just four valence electrons, while those in Group 3A can be stable with six. Transition elements can have expanded octets with more than eight electrons, such as 10, 12, 14, or 16 valence electrons, based on twice their group number.

Free radicals are molecules or ions with an unpaired electron, which can make them highly reactive and potentially damaging to cells. They have an odd number of total valence electrons. To determine the best Lewis dot structure for a radical, one should place the unpaired electron on the element that results in the least formal charges. The formal charge can be calculated using the formula:

group number−number of bonds−nonbonding electrons A Lewis structure with atoms having formal charges close to zero is considered the most stable representation of the molecule. When the total number of valence electrons is an odd number, it often indicates the presence of a radical.

Lewis Dot Structures: Exceptions

Some elements can have less than eight or more than 8 octetal electrons around themselves and maintain stability. Now recall their non octet number of electrons is 2X their group number. So for incomplete octets where they have less than 8 octet electrons around them, for group 2A it would be 2 * 2 which is 4. For group 3A it'd be 3 * 2 which is 6.

So they can have group 2A. Elements can have four electrons, octet electrons and be stable. Group 3A can have six or just multiplying their group number by two. So 5A could have 10 electrons in me. OK, group 6A could have 12, 14, and 16.

So just remember, sometimes the octet rule is broken and it's still OK in terms of the Lewis dot structure.

Lewis Dot Structures: Exceptions Example 1

Here we have to draw the Lewis dot structure for xenon dibromide molecule. So xenon is in Group 8A. It's a noble gas, so it has 8 valence electrons. Bromines in Group 7A, so it has seven, and there's two of them, so we have 22 total valence electrons. Now Xenon will go in the center and here we're going to be connected to our two bromines.

Now remember, your surrounding elements need to follow the octet rule. So we're going to put our electrons around bromines so that they each have 8 total valence electrons, 3 lone pairs around them, totaling 6 electrons. But remember, they're also sharing electrons from the single bond, so that's eight each one has. So that's using up 16 of my total 22 valence electrons. So we have 6 remaining here.

The remaining 6 electrons, we have no choice but to put them around Xenon. So 6 electrons and we separate them evenly as lone pairs and this would be the structure of xenon dibromide molecule. We can see here that xenon has 246810 electrons around it. It's breaking the octet rule because it is an exception.

Now it's ideal non octet number would have been 16, but again that's when it's ideal. Here we just don't have enough electrons to get to that number of 16. Instead, xenon is OK with having 10 electrons around it. But here we're seeing that our central elements are breaking the octet rule, and it's still OK.

Determine the Lewis Dot Structure for the following compound:SOCl₂

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Answer: (A)

Lewis Dot Structures: Exceptions

So some of you may have heard of the term free radicals when discussing health or nutrition. We know that free radicals can damage healthy cells within our bodies. But what is a free radical look like? Well, free radicals are just molecules or ions with an unpaired electron around an element. In this case, we have this unpaired electron on the nitrogen atom.

We're going to say radicals or compounds. Radical compounds always have an odd number of total valence electrons, Nitrogens in Group 5A, so it has five. Oxygens group 6A, so it has six. So this molecule has a total of 11 total valence electrons. Now we're going to say to draw it you place the electron on the element that gives least formal charges.

So if we took the formal charge of nitrogen and oxygen. So remember formal charge equals group number minus. The bonzi element is making non bonding electrons, so nitrogen is in Group 5A so it has five valence electrons. We see it making two bonds and it has three electrons so it equals 0. Oxygen is in Group 6A. We see it making two bonds and it has four electrons that are not bonding, so it is also equal to 0. So both molecules have zero foam charges.

Remember we want to get all our atoms if possible within the Lewis dot structure as close to 0 as possible. This gives us the best representation for a particular molecule. Now again, the giveaway that we're dealing with a radical is when you calculate the total number of valence electrons and you get back in odd value. This is usually a strong indication that a radical may be present, so play around and see what makes the most sense in terms of the free radical compound.

Lewis Dot Structures: Exceptions Example 2

Here it says draw the Lewis dot structure for the radical of nitrogen dioxide. So nitrogen dioxide is a very common example used to talk about radicals. If we look at the total number of valence, we have 5 from nitrogen since it's in Group 5, a 6 * 2 oxygens in Group 6A, so it has six, and there's two of them. So this has a total of 17 total valence electrons. It's an odd number of valence electrons, so that's a strong indication we're dealing with a radical.

We place nitrogen in the center. It forms single bonds to the oxygens initially. Make sure that your surrounding elements follow the octet rule. Right now we have a total of 16 electrons being depicted, leaving us with one electron left. The issue now is that nitrogen is not fulfilling the octet rule. It has 2³5 electrons around it. So remember, when an element is not fulfilling the octet rule, what we can do is make double or triple bonds.

Here we can't make a triple bond because then that would be too many electrons around nitrogen. It can only go up to 8 for the octet rule. So we're just going to use one of the lone pairs on oxygen, either one to make a double bond and in that way nitrogen has 7 electrons around it and that's the best that we can do. This year depicts what the nitrogen dioxide molecule would look like. It is a radical because we have that one lone electron on top of nitrogen.

Draw the Lewis Dot Structure for the radical hydroxide, OH.

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• B.
• C.
• D.

Answer: (B)

Draw the Lewis Dot Structure for POCl₃.

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• D.

Answer: (A)

Metalloids can sometimes adopt the bonding preferences of similar nonmetals. Based on your knowledge of expanded octets, draw the Lewis Dot Structure for the following ion, SiF₆^2–.

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• B.
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• D.

Answer: (B)

Here’s what students ask on this topic:

What are the exceptions to the octet rule in Lewis dot structures?

In Lewis dot structures, the octet rule states that atoms tend to have eight electrons in their valence shell. However, there are exceptions. Elements in Group 2A can be stable with just four valence electrons, while those in Group 3A can be stable with six. Transition elements can have expanded octets with more than eight electrons, such as 10, 12, 14, or 16 valence electrons, based on twice their group number. Additionally, free radicals, which have an odd number of total valence electrons, also break the octet rule.

How do you determine the best Lewis dot structure for a free radical?

To determine the best Lewis dot structure for a free radical, you should place the unpaired electron on the element that results in the least formal charges. The formal charge can be calculated using the formula: $\mathrm{Formal} \mathrm{charge}=\mathrm{Group} \mathrm{number}-\mathrm{number} \mathrm{of} \mathrm{bonds}-\mathrm{nonbonding} \mathrm{electrons}$. A Lewis structure with atoms having formal charges close to zero is considered the most stable representation of the molecule.

What is a free radical in chemistry?

A free radical in chemistry is a molecule or ion with an unpaired electron, making it highly reactive. Free radicals have an odd number of total valence electrons. They can cause damage to cells and are often discussed in the context of health and nutrition. In Lewis dot structures, the presence of an odd number of valence electrons usually indicates a free radical.

Why can some elements have more than eight electrons in their valence shell?

Some elements can have more than eight electrons in their valence shell due to the availability of d-orbitals in their electron configuration. This is common in transition elements and elements in periods 3 and beyond. These elements can expand their octet to accommodate 10, 12, 14, or even 16 valence electrons, based on twice their group number. This expanded octet allows them to form more bonds and achieve greater stability in certain compounds.

How do you calculate the formal charge of an atom in a Lewis structure?

The formal charge of an atom in a Lewis structure can be calculated using the formula: $\mathrm{Formal} \mathrm{charge}=\mathrm{Group} \mathrm{number}-\mathrm{number} \mathrm{of} \mathrm{bonds}-\mathrm{nonbonding} \mathrm{electrons}$. This formula helps determine the most stable Lewis structure by ensuring that the formal charges on all atoms are as close to zero as possible.