Example
Which one of the following molecules contains a triple bond: PF3, NF3, C2H2, H2CO, or HOF?
Explanation
The answer is C
2H
2, which is also known as ethyne. When drawing this structure, remember the rules. Find the total number of valence electrons in the molecule by adding the group numbers of its constituent atoms. So for C
2H
2, this would mean C = 4
2 (since there are two carbons) = 8. Add to this the group number of H, which is 1, times 2 because there are two hydrogens = a total of 10 valence electrons. Next, the carbons are clearly acting as the central atoms since hydrogen can only have two electrons and thus can’t form more than one bond. So your molecule looks like this: H—C—C—H. So far you’ve used up six electrons in three bonds. Hydrogen can’t support any more electrons, though: both H’s have their maximum number! So your first thought might be to add the remaining electrons to the central carbons—but there is no way of spreading out the remaining four electrons to satisfy the octets of both carbon atoms except to draw a triple bond between the two carbons.
For practice, try drawing the structures of the other four compounds listed.
Example
How many sigma (s) bonds and how many pi (p) bonds does the molecule ethene, C2H4, contain?
Explanation
First draw the Lewis structure for this compound, and you’ll see that it contains one double bond (between the two carbons) and four single bonds. Each single bond is a sigma bond, and the double bond is made up of one sigma bond and one pi bond, so there are five sigma bonds and one pi bond.
Exceptions to Regular Lewis Structures—Resonance Structures
Sometimes you’ll come across a structure that can’t be determined by following the Lewis dot structure rules. For example, ozone (O3) contains two bonds of equal bond length, which seems to indicate that there are an equal number of bonding pairs on each side of the central O atom. But try drawing the Lewis structure for ozone, and this is what you get:
We have drawn the molecule with one double bond and one single bond, but since we know that the bond lengths in the molecule are equal, ozone can’t have one double and one single bond—the double bond would be much shorter than the single one. Think about it again, though—we could also draw the structure as below, with the double bond on the other side:
Together, our two drawings of ozone are resonance structures for the molecule. Resonance structures are two or more Lewis structures that describe a molecule: their composite represents a true structure for the molecule. We use the double-directional arrows to indicate resonance and also bracket the structures or simply draw a single, composite picture.
