Bonding Preferences - Online Tutor, Practice Problems & Exam Prep

Understanding bonding preferences is crucial in chemistry, particularly regarding the octet rule and valence electrons. Valence electrons determine an atom's bonding capacity, with each lone pair counting as two electrons and each bond as one. For example, carbon can achieve stability through various configurations, all satisfying the octet rule, but differing in valence electron counts. This variance affects stability, highlighting the importance of recognizing how atoms share or own electrons in bonds and lone pairs to predict molecular behavior effectively.

What is a valence electron?

So now let's talk about probably one of the most helpful topics in this entire chapter, and that's the topic of bonding preferences. This is something that I remember when I was an undergrad and I was taking organic chemistry, I was really confused about because my professor often would just assume that I understood how atoms worked and how atoms bonded. I remember my professor would say, "Oh, nitrogen forms 3 bonds, and oxygen with a positive charge forms 3 bonds." I thought I was supposed to memorize all this stuff. It was really confusing. It turns out that there's really simple logic that we can use to figure out exactly how many bonds every atom wants to have. Once you have this down, you're never going to forget it. So, let's go ahead and move just right along.

Bonding preferences are based on the concept of octet electrons but also based on another type of electron. Let's talk about it. It turns out that there are actually several ways to combine octet electrons in order to satisfy the octet rule for an atom. "Valence electrons" is the name that we give to the octet electrons that the atom actually owns. So remember that we said that atoms can choose to share electrons in bonds or they can choose to have electrons as lone pairs. Both of them are going to count the same in terms of octet electrons because they're surrounding part of the shell. But they're actually going to count differently in terms of the valence count.

The number of valence electrons is going to determine which of the octets you could make. If there are several versions of octets, it's going to determine which of the octets is the most stable. This is the really basic rule. We're going to say that an atom is going to own every lone electron that it has, and it's going to own only one electron for every bond that it has. A really nice easy way to say this is that it's going to own every dot, one electron for every dot, and it's also going to own one electron for every stick. So, I'm just going to say dot equals 1 and also stick equals 1. That's just another way to say it.

What is the difference between valence and octet electrons?

So let's go ahead and do this kind of worked example and you guys can help me figure out like the differences in stability for these. All right. So the first thing I want you guys to do, we just talked about the octet rule. So what I want you guys to do is to figure out how many octet electrons each of these carbons would have. So go ahead and start off from the left right there and go ahead and pick out how many octet electrons does it have? 8. Okay. So let's write that in. Now what I want you to do is look at the next one and say, okay, well, how many electrons does that one have? So let's move to the right and what you're going to find there is that you also have 8. It's different though. It looks a little different, but I have 2 from the lone pair and 2 from each of these bonds. So it's still 8. Now we're going to keep going and what you're going to find is that all of these fulfill the octet rule. They all give carbon 8 electrons. Does that make sense so far? Even though they look radically different, but they still follow the octet rule. So now here's my next question. Do you think that these are all equally stable? Do you think that they could all exist in the same way? It turns out no. Definitely not. It turns out one of these is way more stable than all the other ones. The reason has to do with valence electrons. The way that we count valence electrons was remember we count dots and sticks. So what I want you guys to do now is count what are the valence electrons for all of these. So let's start off with the one on the left. How many sticks does it have? 4. How many dots does it have? 0. So it would have 4 valence electrons. All right. The next one. How many dots does it have? 2. How many sticks does it have? 3. 2+3 is 5. And you could keep going and what you're going to find is that this one has 6 valence electrons and this one has 7 valence electrons. All right. So what we find is that the octet rule is being satisfied with all of these, but they have very different amounts of valence electrons. Okay.

Understanding the hydrogen isotopes.

It turns out that if your octet is filled with all these atoms, the way you determine the one that's most stable is by figuring out what row that atom is in. It turns out that for each of the second row elements, the element is going to prefer to own or it's going to prefer to have a valence determined by its group number. So what that means is that if an atom is in group 4, like carbon, then how many valence electrons is it going to want to have? 4. So this is going to be the most stable arrangement of carbon by far, whereas all these other ones are going to get progressively worse and worse. In fact, some of these don't even exist because they would be so crazy hard to create. They'd be so unstable. Does that make sense? So now what I want to do is extend this because you're like oh, well, Johnny, everyone knows carbon has 4 bonds. Maybe you learned that in gen chem. But now I want to extend this to all the other atoms so that you guys will know for the second row what they look like and what they're always going to want to look like in their neutral bonding preference. All right. So let's go through this really quick. As you can see, I've listed different atoms kind of in order of where they are in the periodic table and we're just going to go ahead and start off with hydrogen. Now I understand that hydrogen is not in the 2nd row, but it's way more common than lithium, so we're going to just work with hydrogen. We're going to replace that one. Hydrogen is in group 1a and that means that how many electrons does it want to have in its octet? Do you guys remember? 2. So that means that how can an atom have an octet? Well, it can either have lone pairs or it can have bonds. So that means the hydrogen can exist in 2 different forms. I could have a hydrogen that has a bond. That would give it its 2 electrons. It would fill an octet, but I could also have a hydrogen that has a lone pair. Do you guys see that? Would both of these fulfill the octet? Yes. They would both fulfill the octet. So now my question is, are both of these equally stable? The way that I find that out is I look at the group number. Well, what's the group number? 1. So how many valence electrons does the first hydrogen have? It has one because I only count one for each stick. The second one, how many does it have? 2. So which one is going to be the most stable? This is going to be the way that hydrogen looks. It will not look like this. That's why every time you see hydrogen, it's always attached with just one stick, with one bond because that's going to be the one that satisfies not only the octet but also the valence of having one stick or one valence electron. Does that make sense? Cool. So now what I want you guys to do down here, now that we've figured that out, is really just write what we have. Like let's write our results. How many bonds does hydrogen like to have? 1. How many lone pairs does it like to have? 0. You guys cool with that? Let's move on. So now let's go to beryllium. So beryllium, do you guys remember how many octet electrons beryllium likes to have? Four. All right. It likes to have 4. So there's going to be actually 3 different ways that we could do this. I'll draw all 3 just so that you guys can kind of realize what that is. We could have beryllium with 2 sticks. We could have beryllium with a stick and a lone pair. Or we could have beryllium with 2 lone pairs. Are you guys getting that? All of these versions would satisfy the octet rule equally. But the way I determine the ones that's most stable is I look at the group number. What's the group number? 2. If I'm in group 2, which of these is going to be the favored arrangement? You guys got it. It's beryllium with the 2 sticks. Because that's the only one that is going to not only fulfill the octet but also fulfill the valence. So that means that beryllium in its bonding preference likes to have how many bonds? How many lone pairs? 0. Okay. So hopefully, that's making sense. Let's move on to the next one. All right. So now we are at boron. Do you guys remember how many electrons, octet electrons boron likes to have? This is from your octet rule. I think we talked about it. 6. How many valence electrons does it like to have? 3. So can you guys predict what I'm going to want to do here? I think I heard it. What you're going to want is boron with 3 bonds. The reason is because that's not only going to fulfill your octet of 6, but it's also going to fulfill your valence of 3. Cool? So that means that now it's going to want to have 3 bonds and 0 lone pairs. Are you guys kind of seeing a pattern here? None of these want lone pairs and they're escalating. So we already got to carbon and just by looking at this pattern, what should the next number be? Well, it should be that I have 4 bonds and 0 lone pairs. And when you draw it out, it actually does come out to that. This carbon wants to have 8 electrons. Remember that carbon, nitrogen, oxygen, and fluorine all want to have 8. So they're all going to stay at 8 and it wants to have 4 valence electrons because it's in group 4. So that actually is the arrangement. So now, let's continue to apply wants to have It wants to have 8. If we were just to apply this pattern, don't draw this yet, but I'm just showing you, what I would do is I would draw a 5 here and a 0 here and I would say nitrogen wants to have 5 bonds and 0 lone pairs. Is there a problem with that? Am I making a mistake? I'm actually making a huge mistake. The reason is that if nitrogen had 5 bonds, would it still satisfy the octet rule? No because remember, each bond counts as 2. So if nitrogen had 5 bonds, it would have 10 electrons and it would break the octet rule and it would be even worse off. It would be super unstable. So that means that somehow I need to get nitrogen to have 5 valence electrons, but still only 8 octet electrons. So that means that out of these electrons, out of these 8 electrons, I need to get nitrogen to own more of the electrons. How can we get it to own more? By adding lone pairs. Hopefully, what you guys maybe came to on your own is that nitrogen, what I'm going to want to do is 3 bonds and one lone pair. Because if I can do that, then what I'm going to get is I have 8 electrons total. 8. And then I have every stick counts as 1. 1, 2, 3. And every dot counts as 1. 4, 5. So now when I go down to my pattern, it's actually going to be that I want 3 bonds and one lone pair. Now this is going to start a new pattern. So for oxygen, oxygen still wants 8 electrons, but now it wants to own 6 of them. It wants even more because it's in group 6. So can you guys think of an arrangement that would work for that? Yeah, you guys are smart. You guys know what's going on now. So it would be 2 bonds and 2 lone pairs. Because now the more lone pairs I have, the more I'm owning. So that means that now this would count as 6 valence electrons, 8 octet, so this would be 2 bonds and 2 lone pairs. You guys can see the pattern here. We can guess what the last one's going to be and that pattern actually does hold. Fluorine would want to have you can see the bonds keep decreasing, so fluorine would have one bond and it would have 3 lone pairs. That actually makes sense because if you give fluorine one bond and 3 lone pairs, what you get is an octet of 8, but a valence of 7. Now guys, I really can't emphasize the value of understanding this enough because these bonding preferences are what a bunch of other topics are built off of. For example, bond line structures, formal charges, all this stuff has to do with your knowledge of being able to know these preferences. Also, it's just going to help you draw compounds later.

Here’s what students ask on this topic:

What is the octet rule and how does it relate to bonding preferences?

The octet rule states that atoms tend to form bonds until they are surrounded by eight valence electrons, achieving a stable electron configuration similar to noble gases. This rule is fundamental in understanding bonding preferences because it helps predict how atoms will bond to achieve stability. For example, carbon, with four valence electrons, will form four bonds to complete its octet. Similarly, nitrogen, with five valence electrons, will form three bonds and have one lone pair. Understanding the octet rule allows us to determine the most stable configurations for molecules, as atoms will prefer arrangements that satisfy this rule.

How do valence electrons influence the stability of a molecule?

Valence electrons play a crucial role in determining the stability of a molecule. Each atom in a molecule aims to achieve a stable electron configuration, often by fulfilling the octet rule. Valence electrons are the electrons in the outermost shell of an atom and are involved in bonding. Lone pairs count as two valence electrons, while each bond counts as one. The distribution of these electrons affects the molecule's stability. For instance, different configurations of carbon can all satisfy the octet rule but have varying numbers of valence electrons, leading to differences in stability. The most stable configuration is typically the one with the optimal distribution of valence electrons.

What is the difference between octet electrons and valence electrons?

Octet electrons refer to the total number of electrons surrounding an atom, including both bonding and lone pair electrons, that fulfill the octet rule. Valence electrons, on the other hand, are the electrons in the outermost shell of an atom that are available for bonding. While octet electrons count all electrons around an atom, valence electrons are specifically those that the atom 'owns' in bonds and lone pairs. For example, in a molecule, an atom may have eight octet electrons but only a certain number of valence electrons based on its bonds and lone pairs. This distinction is important for understanding bonding preferences and molecular stability.

How do you count valence electrons in a molecule?

To count valence electrons in a molecule, follow these steps: First, count the number of lone pair electrons (dots) around the atom. Each lone pair contributes two valence electrons. Next, count the number of bonding electrons (sticks). Each bond contributes one valence electron to the atom. Add these values together to get the total number of valence electrons for the atom. For example, if an atom has two lone pairs (4 electrons) and forms three bonds (3 electrons), it has a total of 7 valence electrons. This method helps determine the atom's bonding capacity and stability within the molecule.

Why are some configurations of atoms more stable than others?

Some configurations of atoms are more stable than others due to the distribution of valence electrons and the fulfillment of the octet rule. Stability is influenced by how well the electrons are arranged to minimize repulsion and maximize attraction between atoms. Configurations that allow atoms to achieve a full octet of electrons are generally more stable. Additionally, the specific arrangement of bonds and lone pairs can affect the overall energy of the molecule. For example, a configuration where atoms have the optimal number of bonds and lone pairs, leading to a lower energy state, will be more stable than one with an uneven or less optimal distribution of electrons.