Question

How many valence electrons are used to make sigma bonds in the molecule C3H6?

Answer 1

There are two compunds with this molecular formula, cyclopropane and propene (or propylene), considered to be structural isomers.

`C_3H_6` has `18` valence electrons, 12 from the C atoms (4 from each) and 6 from the H atoms (1 from each). The Lewis dot structures for both cyclopropane, and propene, are

Notice that cyclopropane has a total of 9 sigma bonds, each of these bonds having 2 shared electrons. So, for cyclopropane, all the molecule's valence electrons are used for sigma bonds.

However, notice that propene has a double bond, which consists of 1 sigma bond and 1 pi bond; this makes the total number of sigma bonds equal to 8. So, for propene, only `8 * 2 = 16` valence electrons are used for sigma bonds, the remaining 2 are used for the pi bond.