Can You Rank These Molecules By Valence Electrons? - Must Know Skills!

Rank each of these molecules by the number of valence electrons each one has. You must be able to do this because it's almost always the first step in correctly constructing Lewis structures. So what we've got to do is go from the molecular formula of the molecule to figure out how many valence electrons we have. And that will depend upon the elements that compose it and the number of each. Let's take C2H4, which is one of the four molecules we have to rank. It's clear that the elements present are carbon, which we have two of, and hydrogen, which there are four. And it just so happens that the group to which an element belongs tells us exactly how many valence electrons that element will have. Hydrogen is in group one, carbon is in group four. Therefore, a single atom of hydrogen has one valence electron and a single atom of carbon has four valence electrons. Here's where it all comes together. We know how many valence electrons there are for one carbon and for one hydrogen, but the molecular formula tells us that we have multiple of each of these. Four times two means that carbon contributes eight valence electrons to the molecule, and one times four means that hydrogen contributes four valence electrons. Eight plus four equals twelve, so that means that C2H4 has twelve valence electrons. And we have to calculate the rest to figure out how to correctly rank and compare each of these. In fact, we can use this equation to find the number of valence electrons for each of our molecules or any molecule where we multiply the quantity and the group number for each element present in the compound, and then we have to add them together. I know it looks confusing, but I'll show you how it works because it's actually pretty simple. We just showed how C2H4 has twelve valence electrons, and with this equation we can prove it. We take the quantity of the number of carbons in this molecule, two, and we multiply it by the group number, four. That tells us how many valence electrons each carbon contributes, and that's eight valence electrons. Whereas hydrogen, there's four of them, and we multiply it by the fact that there's one valence electron per hydrogen. We get four and we add it together, we get twelve. That's what the sigma means. It's a summation. It sums up the quantity group number product for each element. We use the same equation for one of the other molecules we have to rank, and so far it's pretty straightforward. There's one carbon, carbon's in group four, so carbon contributes exactly four valence electrons total. But chlorine, we know there's four of them, so we're going to multiply four by something. But the question is, what group on the periodic table does Cl belong to? We look at the periodic table, and we know that chlorine is in group seven, so a single chlorine has seven valence electrons. Therefore, it's going to contribute twenty-eight valence electrons. Four plus twenty-eight is thirty-two valence electrons for the entire CCL4 molecule. So we know that this has a lot more valence electrons than this, but the question is, where do these other two fall into? One thing that's really interesting is that our remaining two molecules have one nitrogen and three oxygens. Nitrogen's in group five, oxygen's in group six, so each atom is going to have five and six valence electrons, respectively. And using our equation from earlier, one nitrogen times group five means nitrogen would contribute five valence electrons, and our math would indicate that NO3 has twenty-three valence electrons. However, we haven't addressed the charges of these molecules, NO3 plus, NO3 minus, and for clarity's sake, it's plus one and minus one, respectively. So if our summation equation tells us that NO3 without a charge is going to have twenty-three valence electrons, I can tell you that one of these is going to have twenty-two valence electrons, and the other one is going to have twenty-four. But which is which? Well, it turns out that the negative charge means there are more valence electrons, because an electron has a negative one charge. With that, the most accurate and thorough equation for the number of valence electrons in a molecule equals the same as before, but now minus the charge. So applying this to NO3 minus, for example, gives us twenty-four valence electrons as we determined from the previous slot. Here's our final ranking of each molecule by the number of valence electrons. It's worth thinking about what elements are in each molecule and how that might correspond to where it falls within this ranking. Speaking of which, where does boron trifluoride fall within this ranking? Go ahead, try that out, and tell me what you think in the comment section.