in this video we're going to go over lewis structures that are exceptions to the octet rule now if you recall the octet rule is that atoms should have eight electrons around it octet means eight the first example that violates the octet rule is bh3 go ahead and draw the lewis structure of bh3 to draw we need to count the number of valence electrons boron has three and hydrogen has one so three plus three is six now every hydrogen atom needs at least one bond and every bond represents two electrons two four six now there's no way in which we can put eight electrons in boron because we don't have eight valence electrons in this molecule so therefore boron has no choice but to have an incomplete oxide and this is the correct lewis structure of bh3 it has a trigonal planar molecular geometry with a bond angle of 120 and it's just the way it is so not every molecule will obey the octet rule the octet rule is just a general rule but this is one example of a substance that has an incomplete octet it has less than eight electrons now there are some other molecules that have more than eight electrons on the periodic table the second row elements that you'll see often is carbon nitrogen oxygen and fluorine these electro excuse me not electrons but these atoms cannot have more than eight electrons so they cannot have an expanded octet however they can have an incomplete oxide it is possible now the elements below them for example like silicon phosphorus sulfur chlorine these elements they can have what is known as the expanded octet they can have more than eight electrons the reason why nitrogen can have more than eight is because nitrogen is in the second row and in the second row you have the 2s sublevel and you have the 2p sublevel notice that in the second energy level the maximum number of electrons is 8. so that's why second row elements cannot have more than eight electrons it's just not possible phosphorus is in the third row so in the third energy level you have the 3s sublevel the 3p sub level and the 3d sublevel so therefore in the third energy level you can have up to 18 electrons and that's why elements like phosphorus sulfur and chlorine they can have expanded octets so if they're the center atom they can have more than eight electrons sometimes you might see them have ten sometimes even twelve so just keep that in mind so anything that's in the third row or below can have an expanded octet but the second row elements like carbon nitrogen oxygen and fluorine they cannot have an expanded oxide they cannot have more than eight electrons now let's consider an example of an expanded octet pcl5 go ahead and draw the lewis structure of this substance phosphorus has five valence electrons and chlorine has seven five times seven is 35 plus 5 is 40. and to calculate the number of lone pairs it's equal to the valence electrons minus 8n divided by 2. so there's 40 valence electrons and n is the number of atoms that is not a center atom so n is five in this example so eight times five is forty so forty minus forty is zero so there are no lone pairs on a center atom so to draw the lewis structure all we need to do is put the five chlorine atoms around phosphorus because there's no lump here on phosphorus and as you can see phosphorus has more than eight electrons around it because it has five bonds and each bond represents two electrons it has a total of 10 electrons around it so it has an expanded octet so let me give another example let's use icl5 go ahead and draw the lewis structure for this molecule so first let's count the number of valence electrons iodine has seven chlorine has seven five times seven is 35 plus seven that's 42. now the number of lone pairs is going to be the valence electrons which is 42 minus 8n and n is the number of atoms that is not the center atom which is 5. so 8 times 5 is 40 42 minus 40 is 2. so we have one lone pair on iodine the center atom and there's five other chlorine atoms attached to it and so this is the lewis structure of iodine pentachloride it has a square pyramidal molecular geometry and as you can see it has an expanded octet it has 2 4 6 8 10 12 electrons around it now there are some other examples that violate the octet rule and this occurs when a molecule has an odd number of electrons so let's consider nitrogen monoxide nitrogen has five valence electrons and oxygen has six so the total is 11. so how can we draw a lewis structure in which we have 11 electrons the first thing i like to take into account is the number of bonds that certain elements like to form so let me just give you a quick review of how to determine that carbon has four valence electrons nitrogen has five oxygen has six fluorine has seven now elements on the right side of the periodic table they like to acquire electrons so carbon wants to have eight it needs to get four more electrons in order to have eight so carbon likes to form four bonds to get those four electrons that it needs nitrogen has five valence electrons it needs three more to get to eight so it likes to form three bonds to get the three electrons it needs oxygen likes to form two bonds fluorine likes to form one now on the left side these elements like to give away electrons instead of acquiring electrons boron has three valence electrons so it forms three bonds to give away those three electrons beryllium has two valence electrons so it likes to form two bonds to give away those two valence electrons and so knowing that can help us to get a good idea of how to draw the lewis structure for no so now let's go ahead and draw no now nitrogen likes to form three bonds but oxygen likes to form two so this molecule is either gonna have two bonds or three bonds so let's start with two we need a total of 11 electrons right now we have four every bond represents two so now we have six and now we have eight now we have nine now we have ten so who's going to get the last electron there's two ways in which we can draw we can give it to oxygen or we can give the last electron to nitrogen as you can see either way you draw one of these atoms will have an incomplete octet in the first example oxygen has eight electrons two four six eight so its oxide requirements are satisfied but nitrogen has seven two four six seven in the second example nitrogen has eight but oxygen seven two four six seven so regardless of how you structure this molecule you're going to have an element that has an incomplete octet so anytime you have an odd number of electrons the octet rule will be violated there's nothing you could do about it it's just based on the numbers that you have now which way is the best way to draw this structure should oxygen have eight or should nitrogen have eight now which element is more electronegative oxygen is more electronegative than nitrogen electronegativity increases as you go up into the right towards fluorine and because oxygen is closer to fluorine than nitrogen on a periodic table oxygen is more electronegative so oxygen is going to take away that one electron and so this is going to be the most stable lewis structure out of the two that's listed now let's try another example in which we have an odd number of electrons go ahead and draw the lewis structure for no2 so nitrogen has five electrons oxygen has six so this is going to be 12 plus five so that's a total of 17. so chances are nitrogen is going to be the center atom since nitrogen can form more bonds in oxygen oxygen likes to have two but nitrogen likes to have three but when you have an odd number of electrons something is going to be lacking so each bond needs at least one single bond now if we calculate the number of lone pairs let's see what's going to happen so we have 17 valence electrons minus 8 n and this two there's two atoms that are not center atoms divided by two so 17 minus eight times two that's 16 that's going to be one so what we have is half of a lone pair half of a lone pair is one electron so now at this point we have a total of five electrons two four five now we need to get as close to eight as possible so if we add another bond nitrogen has seven and if we had another one it would have nine which that cannot happen nitrogen cannot have nine electrons it can't have an expanded octet so the best that we can do is seven seven is as close to eight as possible and nitrogen can't pass eight so now we need to fill up the other atoms with lone pairs so oxygen has a double bond which means it needs two lone pairs to get to eight electrons two four six eight the other oxygen atom on the right has a single bond so it needs three lone pairs to have eight electrons and so this is the lewis structure of the nitrogen dioxide molecule this is 8 8 that's 16 plus 1 so we have a total of 17 valence electrons and so anytime you have an odd number of electrons typically it's going to violate the octet rule you