Lewis Dot Structures: Exceptions - Video Tutorials & Practice Problems
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Lewis Dot Structure Exceptions happen when the central element violates the Octet Rule.
Lewis Dot Structure Exceptions
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Lewis Dot Structures: Exceptions
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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 2 x, their group number. So for incomplete octets where they have less than 8 octet electrons around them, for group 2 a it'd be 2 times 2, which is 4. For group 3 a it'd be 3 times 2 which is 6. So they can have group 2 a elements can have 4 electrons, octet electrons and be stable. Group 3 a can have 6. We're just multiplying their group number by 2. So 5 a could have 10 electrons and be okay, group 6 a 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.
Group 2A and 3A elements have incomplete octets. Group 5A-8A elements can have expanded octets.
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Lewis Dot Structures: Exceptions Example 1
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Here we have to draw the Lewis dot structure for xenon dibromide molecule. So xenon is in group 8 a, it's a noble gas, so it has 8 valence electrons. Bromine is in group 7 a, so it has 7 and there's 2 of them. So we have a 22 total valence electrons. Now xenon will go in the center, and here we're gonna be connected to our 2 bromines. Now remember, your surrounding elements need to follow the octet rule. So we're gonna 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 that's 8. 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 2, 4, 6, 8, 10 electrons around it. It's breaking the octet rule because it is an exception. Now it's non I 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, z9 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.
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Determine the Lewis Dot Structure for the following compound:SOCl2
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Lewis Dot Structures: Exceptions
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So some of you may have heard of the term free radicals when discussing health and nutrition. We know that free radicals can damage healthy cells within our bodies. But what does 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 gonna say radicals, radicals or compounds, radical compounds always have an odd number of total valence electrons. Nitrogen's in group 5 a, so it has 5. Oxygen's group 6 a, so it has 6. So this molecule has a total of 11 total valence electrons. Now we're gonna say to draw, 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 bonds the element is making, plus nonbonding electrons. So nitrogen is in group 5 a, so it has 5 valence electrons. We see it making 2 bonds and it has 3 electrons, so it equals 0. Oxygen is in group 6 a, we see it making 2 bonds, and it has 4 electrons that are not bonding. So it is also equal to 0. So both molecules have 0 full charges. Remember, we wanna get all our atoms if possible within a 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 an 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.
Radical compounds always have an odd number of total valence electrons.
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Lewis Dot Structures: Exceptions Example 2
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Here it says draw the Lewis Knapp 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 times 2, oxygen's in group 6 a, so it has 6 and there's 2 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 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 1 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 top of
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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, SiF62–.
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