Lesson 4: Chains and names

Skeletal formulas

Section 2 of 4

Drawing every hydrogen gets crowded fast. Chemists use a shortcut you already saw at the end of lesson 1: the skeletal formula.

  • The chain is drawn as a zigzag. Every corner and every end of a line is a carbon atom, unless another atom is written there.
  • Hydrogen atoms on carbon are left out. Each carbon has enough of them to make 4 bonds.
  • Atoms other than carbon and hydrogen are written in, with their hydrogens: OH, NH₂.
  • A double bond is still drawn as two lines.
Butane, C₄H₁₀Four carbon atoms in a straight chain, joined by single bonds, with ten hydrogen atoms around them.HHHHHCCCCHHHHH
Butane drawn as a displayed formula, with every atom.
Butane, C₄H₁₀Four carbon atoms in a straight chain, joined by single bonds, with ten hydrogen atoms around them.
The same butane as a skeletal formula: four carbons, at the two ends and the two corners.

Why a zigzag? In lesson 3 you saw that the bonds around each carbon spread out to about 109.5°. A real carbon chain zigzags, and the drawing looks like that chain seen from the side.

Octane, C₈H₁₈Eight carbon atoms in a zigzag chain joined by single bonds.
Octane, C₈H₁₈, as a skeletal formula. The 3D model shows all the hydrogen atoms the drawing leaves out.
Ethanol, C₂H₅OHTwo carbon atoms in a chain; the second carbon is bonded to an oxygen atom that carries a hydrogen atom, an -OH group.OH
Ethanol as a skeletal formula. The oxygen is written in, with its hydrogen: OH.

Quick check

In a skeletal formula, a corner of the zigzag is a carbon joined to two other carbons by single bonds. How many hydrogen atoms does it have?