Are we still using “hacks?” I found myself as a student really struggling with Lewis structures. Adding all the electrons, formal charge and counting to 8 just didn’t work for me. I found myself using the periodic table to help me draw Lewis structures and I use that same strategy for teaching my students. Let me tell you about it!
It’s about “electron spots”
That’s not a real term, but I also use it when we talk about VSEPR Theory, so I like to use it. Any atom that is not violating the octet rule will have four electron spots. Carbon makes 4 single bonds. Nitrogen will make 3 bonds and have 1 lone pair (4 places for electrons). Oxygen will make two bonds and have two lone pairs (again, 4 places for electrons).
This is the same way I approach VSEPR. Tetrahedral, trigonal pyramidal and bent are all “cousins” because they have the same number of “electron spots.” it helps a bit more when you talk about orbital hybridization, so I find this language helps to make connections between Lewis structures, VSEPR and molecular geometry and orbital hybridization.
But really, it’s all about the VALENCE ELECTRONS
It’s a HUGE concept, especially in high school chemistry. The reason fluorine, chlorine, bromine and iodine are all halogens with similar properties is BECAUSE they have the same number of valence electrons. It’s not the other way around. The VALENCE ELECTRONS are the biggest component of an atom or ions behavior. (Then you can include IMFs – like the difference in the dispersion force among halogens, which distinguishes them from each other.)
But let’s consider NH3 – ammonia. Nitrogen will want to make three bonds. Sometimes that’s three single bonds, other times it’s a triple bond. (In weird cases, it’ll be a double and single…) Students will draw this Lewis structure over and over and over again. Why? Because ammonia is a simple molecule. It hydrogen bonds!
But what if you student had to draw a molecule of PCl3. You and I know they’re both trigonal pyramidal. And we know that because we’ve drawn it over and over and over again. But your newbie student hasn’t.
Memorizing Lewis Structures
Probably not a popular opinion. I have my students memorize a few Lewis structures. But the point is that after memorizing these few, they work FROM THEM to draw new ones. Here’s the list of what I have them memorize:
H2, N2, O2 F2, H2O, CH4, NH3 And that’s all. See if they know H2O, they know that oxygen will form two bonds. That can be a double bond or two singles. Apply that to any molecule that has an oxygen. Or any molecule that that has a sulfur, selenium or tellurium. With some practice, it’s easy as pie!
Hacking Groups to Draw Lewis Structures
Phosphorus is a member of group 15, JUST LIKE NITROGEN. So long as it’s not violating the octet rule, it’ll want to form three bonds and have one lone pair. It has 5 valence electrons afterall.
Your students SHOULD know that members of the same group have the same number of valence electrons and BOND THE SAME WAY. We talk about it with the halogens all the time. So why should Group 15 be any different? I teach my students to draw P, just like they’d draw N.
Now Hydrogen and Chlorine are not members of the same group. BUT they have the same goal. Get one valence electron. And for that reason, they’ll each bond once, forming a single single bond.
So in their Lewis structure of PCl3, they’ll simply swap the N from ammonia for the P, and the Hs in ammonia for the Cls.
Benefits of Hacking
Sure, they may not be able to do octet violators or molecules with resonances with ease. But I’m not talking about those kids or those cases. These are your “run of the mill” Lewis structures. Things that are within the confines of the octet rule – ie, things for first year HS chemistry students.
I’ve found that this is a PERFECT concept to call back on when you start talking about substitutions on molecules. Whether that’s the organic substitution reaction, or a greater discussion piece on pharmaceuticals. One brand’s version of a medicine contains a chlorine and the other maybe uses a bromine in its place. Relate it to the fact that both chlorine and iodine are used as disinfectants. The relationship between H2S and H2O in terms of molecular shape (but their difference in IMF).
REVIEW THE PERIODIC TABLE IN YOUR BONDING UNIT. I teach them back to back, obviously PT comes first. This is a great way to keep those periodic table skills sharp as you move through Bonding and deeper into chemistry. It’s like a cheat code for spiral review.
This isn’t the Primary Way I Teach it
In my lesson on Lewis structures I don’t actually teach it this way. When teaching how to draw Lewis structures, I teach it as if one atom was saying to the other “I’ll share one with you if you share one with me.” By doing this, they learn how the lines and dots are different and what they actually mean. This method is used as the “hack” to help them to draw a greater abundance of Lewis structures quickly and with ease.
