【Daily Question】Theoretical Foundation 2020.07.06
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Multiple-choice question: Among the chemical bonds in the following substances, which one contains only σ bonds? (A) PH3 (B) H2C=CH2 (C) CO2 (D) C2H2 Answer: A Hint: Extra rewards will be given to those who can explain the solution process; the answer can be found in the response
The correct answer is A. Explanation: A sigma bond is formed when atomic orbitals overlap along an axis; it can rotate around this axis without affecting the bond strength or bond angle. It is often described figuratively as a “head-to-head sigma bond””； A bond that forms when the protruding parts of two electron orbitals overlap is a π bond. A π bond cannot rotate around the axis of symmetry, and its bond energy is weaker than that of a σ bond; it is often figuratively referred to as a “side-by-side π bond”. The structural formula of C2H2 is CH=CH, with two pi bonds and one sigma bond in the molecule ； In PH3, the H–P bond is a σ bond ； The C==O in CO2 contains a pi bond ； The C=C bond in H2C2=CH2 contains a π bond.