Shapes of Organic Molecules; σ and π Bonds

2026 Syllabus Objectives

By the end of this topic, you should be able to:

  1. Describe organic molecules as either straight-chained, branched or cyclic
  2. Describe and explain the shape of, and bond angles in, molecules containing sp, sp² and sp³ hybridised atoms
  3. Describe the arrangement of σ and π bonds in molecules containing sp, sp², and sp³ hybridised atoms
  4. Understand and use the term planar when describing the arrangement of atoms in organic molecules, for example ethene

1. Types of Organic Molecules: Straight-Chain, Branched, and Cyclic

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Organic molecules are compounds made up of carbon and hydrogen atoms (and sometimes other elements). The carbon atoms in these molecules can arrange themselves in different ways.

Straight-Chain Molecules

Straight-chain molecules are those where all the carbon atoms are connected to each other in one continuous line, like beads on a string.

Example: Pentane (C₅H₁₂)

  • The five carbon atoms are joined in a single chain: CH₃-CH₂-CH₂-CH₂-CH₃
  • Even though we draw it as a straight line on paper, the actual molecule has a zig-zag shape because of the bond angles (which we'll learn about later)
  • Think of it like a chain where each link connects to the next one in order

Branched Molecules

Branched molecules have a main chain of carbon atoms, but they also have extra groups of atoms sticking out from the sides. These side groups are called branches.

Example: 2-methylbutane

  • The main chain has 4 carbon atoms
  • There's a branch (a -CH₃ group called a methyl group) attached to the second carbon atom
  • It's like a main road with a side street branching off

Cyclic Molecules

Cyclic molecules have their carbon atoms arranged in a ring or loop shape, rather than in a line.

Example: Cyclopentane (C₅H₁₀)

  • The five carbon atoms are connected to form a pentagon-shaped ring
  • Each carbon also bonds to hydrogen atoms
  • Think of it like a circular chain where the ends meet

Important note: These straight-chain, branched, and cyclic molecules are all called aliphatic compounds, as long as they don't contain a benzene ring (a special six-carbon ring you'll learn about later).

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