The number of sigma bonds in
H₃C-C=CH-C≡C-H
is _______________.

Solution

Chemistry ma’am · 4 steps

Every bond between two atoms has exactly one sigma, however many lines you draw for it.

1. The rule. A single bond is one σ bond. A double bond is one σ and one π, and a triple bond is one σ and two π. So count the atom-to-atom links, not the lines.
2. Carbon to hydrogen. The molecule is CH₃-CH=CH-C≡CH. It has 3 C–H bonds on the methyl group, 1 on each of the two sp² carbons and 1 on the terminal alkyne carbon. That makes 6 C–H σ bonds.
3. Carbon to carbon. Five carbons in a chain are joined by 4 links: C–C, C=C, C–C and C≡C. Each link gives one σ bond, so there are 4 C–C σ bonds. The 3 extra lines (1 in the double bond, 2 in the triple) are π bonds.

   Total σ = 6 + 4 = 10

4. So. The molecule has 10 σ bonds and 3 π bonds, so the answer is 10.

The common slip: 13. Counting every line as a sigma bond gives 13, but the extra line in C=C and the two extra lines in C≡C are π bonds.

The idea to keep. Count sigma bonds as the number of pairs of atoms that are bonded together. Extra lines in a double or triple bond only add π bonds.

Answer from the NTA final answer key. Solution by PrepWiser.