A-Level Chemistry October/November 2024 Q3(f)(i): At very high temperatures, phosphorus can form P2 molecules. P2 contains a triple bond,…

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  The P≡P triple bond is composed of one sigma (σ) bond and two pi (π) bonds.

  • Sigma (σ) bond: One σ bond is formed by the direct, head-on overlap of atomic orbitals (e.g., p-orbitals or sp-hybrid orbitals) along the axis connecting the two phosphorus nuclei.

  • Pi (π) bonds: Two π bonds are formed by the sideways overlap of parallel p-orbitals. One π bond forms from the overlap of p-orbitals above and below the internuclear axis, and the second π bond forms from the overlap of another pair of p-orbitals in front of and behind the internuclear axis.

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  How the marks are awarded

  • M1 — Correctly identifying the formation of one sigma (σ) bond through the 'head-on' overlap of orbitals.
  • M2 — Correctly identifying the formation of two pi (π) bonds through the 'side-on' overlap of p-orbitals.

  Common mistakes

  • Confusing the number of bonds, e.g., stating there are two sigma bonds and one pi bond.
  • Reversing the type of overlap, e.g., stating sigma bonds form from sideways overlap and pi bonds from head-on overlap.
  • Failing to specify the number of each type of bond (one sigma and two pi).
  • Incorrectly suggesting that s-orbitals can form pi bonds.

  Examiner tip: For questions on covalent bonding, always associate sigma (σ) bonds with single bonds and head-on overlap, and add pi (π) bonds from sideways p-orbital overlap for any multiple bonds.

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