1. A Review of General Chemistry
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Now sometimes you might draw an organic molecule and you aren't quite sure if it's drawn correctly. Well, that's where formal charge can help. It's used to determine and check to see if you drew your Lewis dot structure correctly. Now some key takeaways from this is that the only allowable formal charges for an element can be either negative 1, 0, or plus 1. So if you took the formal charges of all the elements within your molecule, and they don't give you one of these three numbers, that just means that that might not be the best way to draw it. Now, if you add up all the formal charges in your compound, well that'll equal the overall charge of the compound. Now when it comes to the formal charge formula, we're going to say that formal charge equals valence electrons, which remember is just the group number of your element, minus your bonds that element is making, plus non bonding electrons. Now, notice that this is in parenthesis, so remember your orders of operation to calculate this correctly. And when it comes to non bonding electrons, they they are counted individually. Now, here in this example it says, determine the formal charges of each element with a thionylcyanate ion. Now nitrogen's in group 5 a, so it has 5 valence. Carbon would have 4. Sulfur would have 6. Coming back to nitrogen, we see it making 1, 2, 3 bonds with the carbon, and it has 1, 2 nonbonding electrons. So that would be 0 for its formal charge. Then we have carbon. It's a group 4 acyl 4, minus the 4 bonds it's making total, and it has no non bonding electrons, so it's also 0. Sulfur is in group 6 a, so 6 minus the one bond it's making, and then if we count up each of these electrons individually, it adds up to 6. This would give me minus 1. So we have 0 plus 0 plus minus 1 gives me a minus 1 overall charge for the thiocyanate ion, so this will be minus one. This makes sense because this is a polyatomic ion, which has a charge of minus 1. Right? So this is how we can utilize whole charge to see if a structure makes sense.
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