In this video, we're gonna take a look at the 18 and 16 electron role. Now, remember main group elements want to follow the octet rule because that gives them the ideal number of electrons just like a noble gas. Now, main group elements remember our groups one A 28 A and if we look there are eight slots, so 12345678. That's what that's what explains why eight is the ideal number. In terms of the octet rule, exceptions to this are hydrogen and helium. Helium only has two valence electrons involved with it. Hydrogen only needs to gain one more to have the configuration like helium. Now we're gonna say when it comes to transition metals though transition metal chemistry we use the 18 and 16 electron rule as an indicator for the reactivity of a transition metal. So we're gonna say the most stable transition metal complexes in several cases have electron counts of 18. Okay, so 18 electrons. This trend because we're dealing with 18 electrons is called the 18 electron rule. How do we come up with this number of 18? Well, the most stable number of electrons represent the number of total SP and D electrons together. Remember your ass have to d have 10 electrons max they can hold and P has six. So when we add those up together, two plus 10 plus six, that gives us 18. So that's where this 18 electron rule comes from. And remember when it comes to these transition metals they have number of valence electrons based on the number of S and D orbital electrons. So from three All the way up to 12. Now there are exceptions. Uh 18 for transition metal is great for it to get to a noble gas configuration by incorporating its d orbital's. But there are exceptions to this. We're gonna say exceptions often happen with transition metals from 8 to 11 valence electron groups. So we're talking about the transition metals within this sector and below. Now we're gonna say the tendency of these medals to be happy with 16 electrons is called the 16 electron rule. Now for a lot of the transition metal catalyzed reactions that we're going to see in organic chemistry, the two most common transition metals that do this. 16 electron rule are palladium and nickel. So they could follow the 16 electron rule of course all transition metals would love to get 18 electrons because that would be the best configuration to become more like a noble gas. But palladium and nickel especially are also okay with 16. So we take this into consideration when thinking about transition metals. Whereas main group elements are trying to get eight for the octet rule, transition metals in terms of stability are always aiming for 18 and a few cases where 16 is okay Now that now that we've talked about the 18 and 16 electron role for transition metals will move on to examples in terms of electron counts. So just remember transition metals ideally want 18 electrons. Whereas main group metals, we're used to seeing them ideally wanting eight electrons.