Drawing Atomic Orbitals: Study with Video Lessons, Practice Problems & Examples

To draw atomic orbitals for conjugated molecules, follow two rules: first, the number of atomic orbitals equals the number of conjugated atoms, with one orbital per conjugated atom. Second, identify the pi electron contributions from nonbonding orbitals: empty orbitals and carbocations contribute 0, pi bonds and radicals contribute 1, while lone pairs and anions contribute 2. This understanding is crucial for analyzing molecular structures and their reactivity in organic synthesis, particularly in reactions involving electrophiles and nucleophiles.

Two Rules of Drawing Atomic Orbitals

Now that we understand a little bit about how atomic orbitals can blend together into molecular orbitals, I want to go back to the beginning and make sure that we all understand how to draw atomic orbitals correctly. So let's get started. Thankfully, transforming a conjugated molecule into atomic orbitals only requires 2 steps, and they're super easy, so this lesson should be very easy for you. Rule number 1: the number of atomic orbitals that you have in your molecule should be equal to the number of conjugated atoms that you have. The rule basically states that you should have 1 atomic orbital drawn per conjugated atom. Notice that in this molecule that I have drawn, it's an anion, I actually have 4 atoms, 1, 2, 3, 4. Good? But let's look again at how many of those atoms actually have nonbonding orbitals, have orbitals that are not bonded to atoms. Well, it turns out that one doesn't count because it only has orbitals that are attached to atoms, so that would not be a conjugated atom. The other ones are conjugated though because we know that 2 has an orbital with an electron, 3 has an orbital with an electron, and then an anion. Anytime that you see an anion, that means it's a lone pair with a negative charge. So those are nonbonding orbitals and for every nonbonding orbital or conjugated atom, you would have 1 atomic orbital. So that means that then I would just put 3 atomic orbitals, and this would just basically be for atom 2, atom 3, and atom 4. Easy enough, right? So then, rule number 2 says you need to know what type of pi electron contribution each type of nonbonding orbital will have. So remember we went over the nonbonding orbitals and we said that there are different types, right? Let's start from the beginning. Empty orbitals and carbocations donate 0 electrons because they have 0 electrons inside, right? Pi bonds and radicals donate 1 each because in each situation, there's one electron that's possible to be conjugated. And then finally, a lone pair and an anion, notice that they have 2 electrons in the orbitals, they donate two each. Okay, so what you would do is you would count up the number of atomic orbitals that you have, line them up, and then you would add in the number of pi electrons that are being contributed. In the following examples, we're going to go over some molecules, and we're going to try to draw the atomic orbitals for them.

Supply Atomic Orbitals

Let's do a first. So, it says provide the correct atomic orbitals for the following conjugated molecules. So, how do we start with number 1? How many atomic orbitals should there be on this molecule? Well, even though I see 5 atoms in total, the conjugated part is just this, right? Because those are the only atoms that can resonate. So that means that I should have 1, 2, 3, 4 atomic orbitals, and I'm going to draw them like this. Cool. Now I have to figure out how many electrons are being donated throughout the entire molecule. Okay?

One way to do this is to just count them up the way that I said, which is that every pi bond atom donates 1 electron. Okay. So right now, I have 2 pi bonds, but there are 4 atoms that are inside those 2 pi bonds, right? So that means each atom would donate 1 electron, meaning that I would have 1, 2, 3, 4 electrons. Okay. Now, another shortcut you can take, which might be helpful, is you can also think that a pi bond in total always donates 2 electrons over 2 atoms. So, another way you could think of it is that there are 2 electrons here, 2, 3, 4 electrons. 3 and 4 are there, so there should be 4 in total. Does that make sense? Because you have 2 here and then you have another 2 here, giving you a total of 4 electrons. Okay.

So I got to this answer using my definition, but you can also just use the shortcut that a pi bond always has 2 electrons that are being donated. Cool. So let's go ahead and try to do the second one on your own, and then I'll show you how.

Supply Atomic Orbitals

So for the second one, provide the correct atomic orbitals, what I see is that the conjugated part of the molecule is this, right? So how many atoms is that? It appears to be 1, 2, 3, 4, 5. I have 5, so let's draw that out. 1, 2, 3, 4, 5. Cool. So that means that now I have to count up my pi electrons. Now my pi electrons, we know that double bonds donate 2 each, so I'm going to have 2 here, I'm going to have 2 here. I know that cations donate 0, right? So that means the total sum should be equal to 4. I have 4 pi electrons total. Now I just have to put them in the right spots, and you would actually try to line it up based on the order of these atoms. So what I would do is I would do this: 1, 2, nothing, 3. And what that shows me is that there are 4 pi conjugated electrons inside of 5 atomic orbitals. This would be the correct atomic orbital diagram for this molecule. Cool. Let's keep going.

Supply Atomic Orbitals

So what is the conjugated portion of this molecule? What I see is that it's actually quite long. It's all of this. 1, 2, 3, 4, 5, 6, 7. I'm going to go ahead and write these out: 1, 2, 3, 4, 5, 6, 7. These are all conjugated to each other, so that means that my atomic orbitals should have 7 orbitals in it: 1, 2, 3, 4, 5, 6, 7. Cool. And now we have to put in the correct amount of electrons. We know that each pi bond counts as 2, so that means I would have 2 here, 2 here, 2 here, and then what does the negative charge count as? If you look up on our little chart, it also counts as 2. So how many electrons do we have total? 8. So you have to put 8 electrons into 7 orbitals and the way that we would draw this is to have 1 electron per orbital, except at the very end we're going to put 2 because that's where the anion is. So we would draw it like this: 111111² and that represents our anion. Cool? So, guys, these are our atomic orbitals. That's as easy as it is. I'm glad we practiced this because this is going to be very important to understand how to draw molecular orbitals. So let's go ahead and end this video and go to the next one.

Here’s what students ask on this topic:

How do you determine the number of atomic orbitals in a conjugated molecule?

To determine the number of atomic orbitals in a conjugated molecule, follow these steps: First, count the number of conjugated atoms in the molecule. Each conjugated atom will contribute one atomic orbital. Conjugated atoms are those that have nonbonding orbitals, such as lone pairs, radicals, or pi bonds. For example, in a molecule with four atoms, if three of them have nonbonding orbitals, you will have three atomic orbitals. This is crucial for understanding the electronic structure and reactivity of the molecule.

What is the pi electron contribution of different nonbonding orbitals?

The pi electron contribution of different nonbonding orbitals varies: Empty orbitals and carbocations contribute 0 electrons because they have no electrons. Pi bonds and radicals contribute 1 electron each, as they each have one electron available for conjugation. Lone pairs and anions contribute 2 electrons each, as they contain two electrons in their orbitals. Understanding these contributions is essential for drawing accurate atomic orbitals and predicting molecular behavior in reactions.

Why is it important to know the number of atomic orbitals in a molecule?

Knowing the number of atomic orbitals in a molecule is important because it helps in understanding the molecule's electronic structure and reactivity. This knowledge is crucial for predicting how the molecule will interact in chemical reactions, particularly in organic synthesis involving electrophiles and nucleophiles. It also aids in visualizing molecular orbitals, which is essential for analyzing conjugated systems and their stability.

How do lone pairs and anions contribute to atomic orbitals?

Lone pairs and anions contribute to atomic orbitals by donating 2 electrons each. This is because both lone pairs and anions have two electrons in their nonbonding orbitals. These contributions are significant when drawing atomic orbitals for conjugated molecules, as they affect the overall electron distribution and reactivity of the molecule. Understanding these contributions helps in accurately predicting molecular behavior in various chemical contexts.

What is the significance of pi electron contributions in drawing atomic orbitals?

The significance of pi electron contributions in drawing atomic orbitals lies in their impact on the molecule's electronic structure and reactivity. Pi electrons, contributed by nonbonding orbitals such as lone pairs, radicals, and pi bonds, determine the distribution of electrons in the molecule. This distribution is crucial for predicting how the molecule will interact in chemical reactions, particularly in organic synthesis. Accurate pi electron contributions help in visualizing molecular orbitals and understanding the stability and reactivity of conjugated systems.