Free Sigma and Pi Bonds MCQs with Answers

1. A sigma bond is formed by

• A. sideways overlap of two p orbitals
• B. head on overlap of two orbitals along the axis joining the nuclei
• C. the transfer of electrons from one atom to another
• D. attraction between two ions

Explanation: End on overlap concentrates electron density directly between the nuclei, which makes a sigma bond stronger than a pi bond and allows free rotation about it. Sideways overlap of parallel p orbitals gives a pi bond, with density above and below the internuclear axis. Every single bond is a sigma bond, and every multiple bond contains exactly one.

2. The number of sigma and pi bonds in a molecule of ethyne, C2H2, is

• A. three sigma and two pi
• B. two sigma and three pi
• C. five sigma and no pi
• D. four sigma and one pi

Explanation: The two carbon to hydrogen bonds are sigma, and the carbon to carbon triple bond consists of one sigma plus two pi bonds, giving three sigma and two pi in total. The rule to apply is that a single bond is one sigma, a double bond one sigma and one pi, and a triple bond one sigma and two pi. Counting them correctly is what makes the hybridisation obvious as well.

3. Rotation about a carbon to carbon double bond is restricted because

• A. the sigma bond is too strong
• B. the atoms are too heavy
• C. turning one carbon would break the sideways overlap of the pi bond
• D. double bonds are ionic

Explanation: The pi bond depends on two parallel p orbitals overlapping sideways, and rotating one carbon by ninety degrees would destroy that overlap, which costs far more energy than is available at room temperature. This rigidity is what makes cis and trans isomerism possible in alkenes. Single sigma bonds rotate freely because their overlap is symmetrical about the axis.

4. In the ethene molecule, the carbon to carbon double bond consists of

• A. two sigma bonds
• B. two pi bonds
• C. one sigma bond formed by sp2 overlap and one pi bond formed by sideways overlap of unhybridised p orbitals
• D. one ionic and one covalent bond

Explanation: Each carbon uses three sp2 hybrids for sigma bonds to two hydrogens and to the other carbon, and the leftover p orbital on each overlaps sideways above and below the plane to give the pi bond. This is why the molecule is flat and why rotation is blocked. Every multiple bond follows the same pattern of exactly one sigma plus the remainder as pi.

5. A coordinate or dative covalent bond differs from an ordinary covalent bond in that

• A. both shared electrons come from the same atom
• B. no electrons are shared
• C. it is much weaker than a normal covalent bond
• D. it involves the transfer of electrons

Explanation: One atom supplies the whole electron pair while the other provides an empty orbital, as when ammonia donates its lone pair to a hydrogen ion to form the ammonium ion. Once formed, the bond is indistinguishable from any other covalent bond, which is why all four N-H bonds in ammonium are identical. Complex ion formation depends entirely on this type of bonding.

6. The number of sigma and pi bonds in a molecule of carbon dioxide is

• A. two sigma and two pi
• B. four sigma and no pi
• C. two sigma and four pi
• D. one sigma and three pi

Explanation: Each of the two carbon to oxygen double bonds contributes one sigma and one pi bond, giving two of each in total. The rule that every double bond is one sigma plus one pi applies universally. A triple bond would contribute one sigma and two pi.

7. A pi bond is weaker than a sigma bond because

• A. it involves fewer electrons
• B. sideways overlap of p orbitals is less effective than head on overlap
• C. it is formed between different elements
• D. it has no electron density

Explanation: Sideways overlap concentrates less electron density between the nuclei than end on overlap does, so the pi bond is easier to break, which is why alkenes are far more reactive than alkanes. Both bond types involve two electrons. This is also why the pi bond of a double bond is the one that opens during addition reactions.

8. In the ammonium ion, all four nitrogen to hydrogen bonds are identical even though one was formed as a dative bond because

• A. the dative bond breaks immediately
• B. once formed, the origin of the electron pair no longer affects the bond
• C. nitrogen changes its hybridisation
• D. one hydrogen leaves the ion

Explanation: A bond is defined by the shared pair between the two nuclei, not by which atom supplied the electrons, so experimentally the four bonds have the same length and strength. The nitrogen is sp3 hybridised and the ion is tetrahedral. This is a favourite examination point because the formation and the final structure are so easily confused.

9. In propene pi-bond is formed by sideway overlap of:

• A. s-orbitals
• B. p-orbitals
• C. sp3 hybrid orbitals
• D. sp2 hybrid orbitals

Explanation: Each doubly bonded carbon uses three sp2 hybrids for its sigma bonds and keeps one unhybridised p orbital, and the sideways overlap of those two p orbitals above and below the plane forms the pi bond. Hybrid orbitals point along the internuclear axis and so can only make sigma bonds. This is why rotation about the double bond is blocked.

10. The theories which explain the formation of sigma and pi bond are EXCEPT:

• A. MOT
• B. VBT
• C. VSEPR
• D. CFT

Explanation: Crystal field theory deals with how the d orbitals of a transition metal ion are split by the surrounding ligands, so it explains colour and magnetic behaviour in complexes rather than the formation of sigma and pi bonds. Valence bond theory and molecular orbital theory both describe those bonds directly through orbital overlap. VSEPR predicts shape from electron pair repulsion and so also concerns bonding.