Which of the following molecules contains only $\sigma$ bonds?

• A. ${\mathrm{CO}}_2$ (carbon dioxide)
• B. $N_2$ (nitrogen gas)
• C. ${\mathrm{CH}}_4$ (methane)
• D. $C_{2}H{4}_{}$ (ethylene)

Answer: (C)

Verified step by step guidance

1. Recall that a sigma (\sigma) bond is a single covalent bond formed by the head-on overlap of atomic orbitals, and it is the strongest type of covalent bond. Pi (\pi) bonds are formed by the side-to-side overlap of p orbitals and usually accompany sigma bonds in double and triple bonds.
2. Analyze each molecule's bonding: Carbon dioxide (CO\_2) has two double bonds between carbon and oxygen, each consisting of one sigma and one pi bond.
3. Nitrogen gas (N\_2) has a triple bond between the two nitrogen atoms, which includes one sigma bond and two pi bonds.
4. Methane (CH\_4) has four single bonds between carbon and hydrogen atoms, and all these bonds are sigma bonds because they are single bonds formed by head-on overlap.
5. Ethylene (C\_2H\_4) has a double bond between the two carbon atoms, which consists of one sigma bond and one pi bond, so it does not contain only sigma bonds.