I have to admit something. I don’t like using puzzle pieces for molecular compounds. There I said it. It’s cute for methane and ammonia, but then it breaks down for carbon dioxide and nitrate ion. Below I will outline my method that has led to a high success rate.
1. Calculate the number of bonds
2. Attach all outer atoms up to the bond number
3. Give all outer atoms Noble Gas configuration
4. Check the “has” number
5. Check for resonance
Below I will give a few hints as to how to use each step.
1. Calculate the number of bonds
I use a redneck equation to calculate bonds. (Wants – Has)/2 = bonds
This simple equation calculates the total number of bonds within the molecule. (Since I use this method with my first year students, we are still not breaking the octet rule. Obviously, this equation falls short with xenon hexafluoride.) The “has” number is the number of valence electrons according to the column on the periodic table. The “wants” number is the number of valence electrons they would have if they reached Noble Gas configuration, which is what they “want” to do to become stable. Therefore, nitrogen has 5 and wants 8. In fact, all elements that I use in first-year chem want to have 8 valence electrons except for hydrogen that has a Noble Gas configuration with only 2.
If there are multiple of the same atom, you will have to multiply the has and wants number by the number of atoms. For instance in phosphorus trifluoride, the has calculation would be 5 + 3(8) = 32. The wants calculation would be 8 + 3(8) = 26
32-26 = 6. Then 6/2 = 3 bonds.
Ions affect the “has” number. If you are drawing nitrate with a charge of 1-, there will be an additional electron added to the “has” number.
2. Attach the outer atoms up to the bond number
In order to attach the outer atoms, we need to know which is the central atom. My rule is that the central atom is the least electronegative non-hydrogen. While I know there can be some exceptions to this (nitrogen triiodide), it works the vast majority of instances. Then students attach the outer atoms including multiple bonds if they need to reach the bond number calculated in the first step.
Obviously, hydrogen can’t receive multiple bonds, but the kids can forget that. Reinforce that the Noble Gas configuration of hydrogen has two valence electrons.
3. Give all outer atoms Noble Gas configuration
Since the outer atoms are the most electronegative, they get first crack at valence electrons. Draw enough pairs to get the outer atoms up to 8. If hydrogen is in the molecule, it is stable with the two electrons found in the bond and don’t get extra pairs. You will have to train your kids on recognizing that a triple bond represents 6 shared electrons so only a single pair of electrons is required.
4. Check the “has” number
Count how many electrons are drawn, including those in bonds. Check the “has” number. If the number drawn and the has number are the same, then the central atom will get no nonbonding pairs. However, if they have not drawn electrons up to the “has” number, all the rest of the electrons will go as pairs on the central atom.
5. Check for resonance
I have two simple qualifiers for identifying molecules with resonance.
#1. Does the molecule have multi bonds?
#2. Does the multi bond have somewhere to go?
If the answer to both of these qualifiers is yes, then resonance should be included in the drawing.
It’s pretty simple. It covers the vast majority of molecules in first year chemistry. And I didn’t have to pick up scraps of paper from a hundred kids cutting out a thousand puzzle pieces only to realize that we can’t always use them. That’s a win in my book.
Try unique ways to connect your curriculum to your students. Use chemistry-focused classroom games. Up & Atom teaches moles. trendy covers periodic trends. And CHeMgO gives reps on writing ionic formulas. Add them to your classroom today!
Comments