Lewis Dot Structures: Exceptions (Simplified) - Online Tutor, Practice Problems & Exam Prep

Some elements can have fewer or more than 8 electrons in their valence shell and still be stable, breaking the octet rule. For example, group 2A elements can have 4 electrons, while group 3A can have 6. Free radicals, which are molecules with an unpaired electron, always have an odd number of total valence electrons. For instance, nitrogen monoxide has 11 valence electrons, confirming its status as a radical. Understanding these concepts is crucial for grasping molecular stability and reactivity.

Lewis Dot Structures: Exceptions (Simplified) Concept 1

Some elements can have less than 8 or more than 8 octet 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'd be \(2 \times 2\), which is 4. For group 3A it'd be \(3 \times 2\), which is 6. So, group 2A elements can have 4 octet electrons and be stable. Group 3A can have 6. We're just multiplying their group number by 2. So, group 5A could have 10 electrons and be okay, group 6A could have 12, 14, and 16. So, just remember, sometimes the octet rule is broken, and it's still okay in terms of the Lewis dot structure.

Lewis Dot Structures: Exceptions (Simplified) Example 1

Here we have to draw the Lewis dot structure for the xenon dibromide molecule. So, xenon is in group 8A; it's a noble gas. It has 8 valence electrons. Bromine is in group 7A, so it has 7, and there are 2 of them. So, we have a total of 22 valence electrons. Now, xenon will go in the center, and here we're going to be connected to our 2 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. Three lone pairs around them totaling 6 electrons. Remember, they're also sharing electrons from the single bond, so each one has 8. 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 a xenon dibromide molecule. We can see here that xenon has 2, 4, 6, 8, 10 electrons around it. It's breaking the octet rule because it is an exception. Now, its 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 okay with having 10 electrons around it. But here we're seeing that our central elements are breaking the octet rule, and it's still okay.

Determine the Lewis Dot Structure for the following compound:SOCl₂

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Lewis Dot Structures: Exceptions (Simplified) Concept 2

When we discuss electron molecules, we typically talk about free radicals. Now free radicals are molecules or ions with a single unpaired electron around an element. We're going to say that these radical compounds or radical molecules always have an odd number of total valence electrons. What's important to understand here is that to draw them, we place the electron on the element that is less electronegative, except in the case of hydrogen itself. That's because if we show a similar electron around hydrogen, it would be breaking the octet rule, or in this case, the duet rule where it wants only 2 electrons around it.

So if we take a look here at this molecule of nitrogen monoxide, we have a lone electron here that's unpaired on the nitrogen. Nitrogen is less electronegative than oxygen, so that's why it has the lone electron. And how do we know that nitrogen monoxide is a radical compound? Again, we would look at the total number of valence electrons and see if it's an odd number. Nitrogen is in group 5a, so it has 5 valence electrons. Oxygen's in group 6a, so it has 6 valence electrons. So the total number of valence electrons for nitrogen monoxide is 11 electrons. It's an odd number and that's why we have a radical molecule or compound in this case. So just keep in mind when we deal with these radical or free radical compounds, this is what you need to be on the lookout for to determine if it is a radical or not.

Lewis Dot Structures: Exceptions (Simplified) 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 electrons, we have 5 from nitrogen since it's in group 5A, 6×2, oxygen's in group 6A, so it has 6 and there are 2 of them. 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 1 electron left. The issue now is that nitrogen is not fulfilling the octet rule. It has 2, 3, 5 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'd be too many electrons around nitrogen. It can only go up to 8 for the octet rule. So, we're just gonna 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 here depicts what the nitrogen dioxide molecule would look like. It is a radical because we have that one lone electron on nitrogen.

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

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Draw the Lewis Dot Structure for POCl₃. 

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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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Here’s what students ask on this topic:

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

Some elements can have fewer or more than 8 electrons in their valence shell and still be stable, breaking the octet rule. For example, group 2A elements can have 4 electrons, while group 3A can have 6. Additionally, elements in groups 5A and 6A can have 10, 12, 14, or even 16 electrons. These exceptions occur because the elements can either have incomplete octets or expanded octets, depending on their group number and the availability of d-orbitals for bonding.

How do you determine if a molecule is a free radical?

To determine if a molecule is a free radical, check if it has an odd number of total valence electrons. Free radicals are molecules or ions with a single unpaired electron. For example, nitrogen monoxide (NO) has 11 valence electrons: 5 from nitrogen (group 5A) and 6 from oxygen (group 6A). Since 11 is an odd number, NO is a free radical. The unpaired electron is typically placed on the less electronegative element, except for hydrogen, which follows the duet rule.

Why can some elements have more than 8 electrons in their valence shell?

Some elements can have more than 8 electrons in their valence shell due to the availability of d-orbitals for bonding. This is common in elements from period 3 and beyond in the periodic table. For instance, sulfur in SF₆ has 12 valence electrons. These expanded octets occur because the d-orbitals can accommodate additional electrons, allowing the element to form more bonds and achieve greater stability.

What is the significance of free radicals in chemistry?

Free radicals are significant in chemistry because they are highly reactive due to the presence of an unpaired electron. This reactivity makes them important in various chemical reactions, including combustion, polymerization, and biological processes like DNA damage and repair. Understanding free radicals helps in studying reaction mechanisms and developing antioxidants that can neutralize these reactive species.

How do you draw the Lewis dot structure for a molecule with an incomplete octet?

To draw the Lewis dot structure for a molecule with an incomplete octet, first determine the total number of valence electrons. Then, arrange the electrons to satisfy the octet rule for as many atoms as possible, starting with the most electronegative atoms. For example, in BF₃, boron (group 3A) has 3 valence electrons, and each fluorine (group 7A) has 7 valence electrons. The total is 24 valence electrons. Boron will have only 6 electrons around it, forming three single bonds with fluorine atoms, resulting in an incomplete octet for boron.