Octet Rule

This might be the most important rule in all of undergraduate chemistry! Let’s start from the beginning and go from there.  

Nobel Gas Configuration

Concept: How Nobel gases are related to the octet rule.  

2m
Video Transcript

All right guys, now what I want to do is introduce probably the most important rule in all of chemistry and that's the octet rule. So let's just jump straight into it.
The octet rule is based on the idea that all atoms on the periodic table eventually want to reach what's called the noble gas configuration. The noble gas configuration just means that they look like noble gases in terms of the amount of electrons that they have. Just so you guys remember, the noble gasses are these pink-colored atoms right there. The noble gasses are the most stable atoms in the entire periodic table. So you can think of all the other atoms as just trying to get to that red state in terms of the stability of the noble gasses.
Actually, this tendency to gain or lose electrons in order to reach that configuration is what's known as the octet rule. So when I say octet rule, all I'm really talking about is how can this atom become like a noble gas in terms of electrons.

The tendency for atoms to gain or lose electrons to reach their Nobel gas configuration is known as the Octet Rule

LCAO of Nobel Gases

Concept: LCAO model proves why Nobel gases are so stable.   

4m
Video Transcript

So, it turns out that in order to prove that noble gases are so stable we can actually use the molecular orbital theory that I was teaching you guys before so let's go ahead before I talk about the periodic table more let's go ahead and go down to this linear combination really quick and I'm going to show you guys why noble gases are so stable. By the way think about it this way if an atom is stable is it going to want to form a bond or not form a bond? The answer is that it doesn't want to for, a bond the reason is because if it's stable by itself it doesn't need anyone else that's like a person that's already financially stable, right? You have your only Pimpass apartment you're not going to like want to get a roommate, right? You're already like balling so why would a noble gas get a roommate and that's exactly the thing it's not going to let me show you why so in this case I'm going to use helium which if you notice helium is one of the noble gases and I'm going to show you guys how it's a little bit different, so remember that these were our atomic orbitals I'm still using 1SA 1SB because these are still 1S orbitals remember that 1S orbitals can hold two electrons how many electrons does each of these helium atoms have? Well according to the atomic number what's the atomic number? 2 so it should have 2 electrons so 1SA should have 2 electrons 1SB should have 2 electrons, alright? So, both of these atomic orbitals are already going to be full right off the bat and that's what I've drawn that here those are the two atoms just nonbonding if they're non-bonding they just wouldn't interact and they'd just be full already. Now what happens if we try to make these interact with the bonding or constructive it interference, OK? What's going to happen is that 2 of these electrons are going to jump down to a lower energy state, OK? And they're going to fill the Sigma orbital I mean they're going to fill the Sigma molecular orbital, OK? So that's good the problem is that I still have 2 electrons left that need to go in another molecular orbital because you can't just combine some of them you have to combine all of them so then it's going to have to jump up to the next energy state because remember that Pauli exclusion principle says you can only fit 2 in each and then after the Aufbau principal says they need to jump up to the next energy level so what that means is that two of these electrons are also going to have to go into that antibonding orbital, do you think that's going to be stable? Not at all that's going to cancel out the stability that was gained from the bonding orbital so basically the bonding orbital is going to be cancelled out by the full antibonding orbital, does that make sense? So, this is really bad so it turns out that I think back in like the 90s they actually did make finally they got helium to bond, OK? It took forever you know how much energy it saved? 0.001 kilojoules per mole, alright? So, what that shows you is that really there is no... Basically, no benefit to Helium interacting through a bonding or through a bombing regular orbital so it's not going to and what you find is that if you do find helium like in a balloon or in outer space wherever, helium will be found with just it in one atom it won't be found as He2 it would just be found as Helium, alright? So, it just shows you how in real life they don't like to bond together, alright?

The LCAO of He2

Science-y Version: Helium would have to fill an anti-bonding molecular orbital in order to bond to itself, which undoes the extra stability provided by the filled bonding orbital, so it has no reason to make a bond.

Basic Version: Nobel gases don’t like to bond to anything!  

Periodic Table Patterns

Concept: The most important parts of the periodic table for organic chemistry.   

3m
Video Transcript

Let's go back up to the periodic table. Now notice that I called this the organic table of elements that's because I kind of tricked this periodic table out to help you guys a little bit. I just want to introduce – just refresh you guys on how to read a periodic table and show you some special features that I put into this one.
First of all, notice that the periodic table has like 118 elements. There's way too many. Organic chemistry is the study of life, the chemistry of life. Living systems are usually not made out of heavy metals. You don't have like a plant that's made out of a bunch of like mercury and Einsteinium and radioactive things. It's made out of like light-weight atoms. It's usually made out of the first and second row elements or first and second period.
So, we're really going to focus on the ones that I shaded as bright, so you see all the bright ones on the screen up there? Those are the ones you want to focus on. I would recommend knowing them. I would recommend being familiar with where they are on the periodic table, so just a heads up. But all these other ones that I grayed out, don't worry about those so much. I'm never going to ask you – your professor's not going to ask you to know very specific things about them.
In terms of metals, metalloids, nonmetals. Don't worry about that too much. That's more inorganic chemistry and gen chem. In organic, we don't really care about categorizing atoms so much. But what really is important is knowing the groups that these atoms are in and knowing the periods that they're in. Remember that period is just the same thing as saying row, so if I were to say row, that means period. Group is the same thing as saying column. So if I ever say column 7, that means group 7.
It's just important to know that you've basically got groups 1 through 8. Some of you guys might be wondering what happened down here. Why do these not get letters? It's because these actually do get numbers, but they're the B's. So this would be like 1B, I mean – yeah. Exactly. It would be 1B, 2B, etcetera. So we don't really worry about those, we just worry about the As.
That's something to keep in mind. I will be referring to these group numbers a lot. I'll just say group 7, group 6, group 5, stuff like that. You should be familiar with what atoms are in which group. Don't worry, by doing lots of orgo, you'll get very familiar with it.

  • Only worry about the elements on the first 2 or 3 rows. Living systems don’t have a lot of heavy metals!
  • Periods = Rows, Groups = Columns.
  • Remember your Group 1A through 8A elements. We will practice memorizing the top atom of each group. 

Octet Rule Details

Concept: The octet rule.

3m
Video Transcript

So now we just want to go over some really, really quick rules and these are going to be the rules that we're going to use for the octet rule. Atoms can satisfy their octet by forming chemical bonds or by possessing lone pairs. What that basically means is that remember that I said atoms want to either lose electrons or gain electrons in order to fit the noble gas configuration.
Well, if you are losing electrons, that means you're going to be sharing electrons with other atoms. That would be a bond. If you are gaining electrons, that means you're going to be taking on more electrons, so you might have a lone pair on the atom. Those electrons are called octet electrons, so keep in mind what an octet electron is.
Basically, let's talk about the first row elements: hydrogen, helium, and lithium. Hydrogen, helium, and lithium are so small that they're usually only going to have just that 1s orbital and it can only have two electrons. Basically, what that means is that they're going to prefer to only have two octet electrons so that they can become like helium.
So hydrogen is going to want to gain an electron. Lithium is going to want to lose an electron so that they can both be like helium. And they're both going to want – basically, all of these are going to want to have two octet electrons. Cool, so far? This is also known in some books as the duet rule. But the duet rule is the same thing as the octet rule. As long as you think of noble gas, it's the same thing.
So then we have our second row elements. So this is first row or first period you could think of. Second row elements are carbon, nitrogen, oxygen, fluorine. These are all going to prefer to possess eight octet electrons that's why we use the name octet rule common.
Now it turns out that there's an exception to that. The atoms that are smaller than carbon, are going to prefer to possess less than eight electrons. The reason is because they don't have that many orbitals, so it's actually difficult for them to accommodate eight electrons. So Be or beryllium is going to prefer to have four and boron is going to prefer to have six. These are just things you need to know. You just need to memorize that.
Then finally, we have our third row or third period elements that are going to be able to form what's called expanded octets. This is what an expanded octet is. We talked about s orbitals and we talked about p orbitals. But it turns out that phosphorus and sulfur are so big that the p orbitals are going to get filled up. Do you know what comes after the p's? The d orbitals. We're not even going to talk about d orbitals in this class. What you should know that d orbitals can also hold extra electrons.
So what's going to happen is that phosphorus and sulfur can choose to hold more than eight electrons if they want to. So phosphorus is going to be able to hold ten electrons and sulfur is going to be able to hold twelve electrons.

  • First-row elements (H, He, Li) will prefer to possess 2 octet electrons
  • Second-row elements (C, N, O, F) will prefer to possess 8 octet electrons
  • Atoms smaller than carbon will possess less than 8 electrons: (Be) = 4 and (B) = 6
  • Third-row elements may form expanded octets that can hold up to (P) = 10 and (S) = 12 electrons.

Octet electrons is the name we give to ALL electrons that surround an atom. These help the atom reach its Nobel gas configuration.

  • Bonds = 2 Octet Electrons
  • Lone Pairs = 2 Octet Electrons

It seems like a lot to memorize, so let’s just knock out some practice problems. 

Problem: PRACTICE: Analyze the following molecules. Indicate ALL atoms that are in violation of the octet rule.

2m

From now on let’s just agree that hydrogens with 1 bond, and carbons with 4 bonds follow the octet rule, instead of multiplying them out every time. 

Problem: PRACTICE: Analyze the following molecules. Indicate ALL atoms that are in violation of the octet rule.

2m

You’ll keep seeing the octet rule a bunch more in this chapter. Let’s move on.