M.Ed.,San Francisco State Univ.
Jonathan has been teaching since 2000 and currently teaches chemistry at a top-ranked high school in San Francisco.
Here are some tips and tricks for drawing a Lewis Electron Dot Diagram. Here are the tricks.
Total up your valence electrons in your molecule or your ion. Then start with all single bonds. Follow the octet or duet rule. Octet rule works for groups 4A, 5A, 6A and 7A and sometimes 8A.
The duet rule works with only group 1A. And then exceptions are for groups 2A, which only makes two pairs, 3A which makes 3 pairs. And then anything that breaks the octet rule with 5 pairs, 6 pairs of electrons, or 7 pairs of electrons, quite possible it'd be is highly unlikely. And then you also have other ions with extra lone pairs that you have there. Let’s take 8A out just to make things easier, because usually, sometimes it does, sometimes it doesn’t. Anyways, here are a couple of examples.
If you have O2, oxygen gas, you total the number of valence electrons. You have 6 times 2 equals, so you have 12 electrons total. So we did that. Start with all single bonds; so I have an O and an O, I have 2O's and then follow the octet or duet rule. So I have one bond here that means I need 6 more around each of the oxygens. Count them up, and here I have 2, 4, 6, 8, 10, 12, 14. I have 14. Do I need that? No, so that one’s wrong.
So we go on to the next one. We have 14, that means we have 2 too many electrons, because we need 12. So what we’ll do is that now we’ll try a double bond. So now I follow the octet rule, I already have 2, 4 in the bond, so 6 and then 8. Count them up, 2, 4, 6, 8, 10, 12, and I have the correct number. So that one’s correct.
Let’s try another one. Say if I have SF6. So we’ll try Sulphur Fluoride. So we count them up; Sulphur has 6, fluorine has 7 and there is 6 of them. So then we have 48 electrons total. We have sulphur, and we have 6 fluorines so we try make them all single bonds. So I have F, F, F, F, F, F. And then we also want to follow the octet rule. So we follow the octet rule for the fluorines. The sulphur already broke the octet rule, if you notice. And so that’s okay because its in the third period below and so it has a special case. We count them up. Each fluorine has 8 total so 8 times 6 is 48, so that’s correct.
Let’s try HCN. H has 1, Carbon has 4, Nitrogen has 5, 1 plus 4 plus 5 equals 10 electrons total. That’s what we have to play with. We’ll try all single bonds. H - C – N. Hydrogen follows the duet rule so that one already has 2. So carbon would need 4 more to make an octet. Nitrogen would need 6 more to make an octet. Count them up, 2, 4, 6, 8, 10, 12, 14. We have too many.
So that means that well, since hydrogen always follows the duet rule, that one’s already correct. Let’s try H – C and then double bonded to the N. Follow the octet rule, count them up, that’s 12. So almost there. The last one H – C, instead of a double bond, let’s try a triple bond now, see if it works. Carbon has 2, 4, 6, 8, around it, Nitrogen needs one more and that’s 10, so that’s correct.
If you take a look here, the pattern is if you have too many electrons, you can always add a bond and then take away two electrons from each of the adjacent atoms, and then you’ll basically take away two. That’s how we got from 14 down to 12 by adding the double bond, and how we got from 12 to 10 by doing the triple bond.
Last one; special case. Say if I have NH4+. Don’t forget the charge, the charge affects it. I have 5 from the Nitrogen, I have 1 from the Hydrogen, times 4 of them and then the plus tells me I have one less electron. So I’ll subtract one. That means I have 8 electrons total. Take my nitrogen and my hydrogens, so I have 4 hydrogens and then I count then up, 2, 4, 6, 8. Then don’t forget to show the charge. Use brackets and then put the charge on the outside, just like it is on the chemical formula.
With the charge, remember a positive charge subtracts that number of electrons. A negative charge adds that number of electrons. Neutral obviously would be no change. Hopefully these tips and tricks, this pattern helps you figure out how to draw a Lewis Dot Diagram a lot easier and a lot faster. Practice, practice, practice that really helps you out. Have a good one.