Key structures for this topic — drawn live.
What a bond-line structure is
A bond-line structure (also called a skeletal or line-angle formula) is the shorthand organic chemists use to draw molecules quickly. Instead of writing out every C and H, you draw only the bonds between carbon atoms as zig-zag lines. It looks sparse at first, but it actually contains all the information you need — you just have to know the two rules for reading it.
The molecule is drawn as a chain or ring of lines. The zig-zag isn't decorative: each change in direction marks a carbon atom. Drawing carbons in a zig-zag (rather than a straight line) reflects the real ~109.5° tetrahedral angles at sp3 carbon and keeps atoms from overlapping visually.
Rule 1: carbons are at ends and vertices
Every place a line ends, and every corner where two lines meet, is a carbon atom. You never write the letter "C" for these. For example, a simple zig-zag with four line segments contains five carbons: one at each of the three internal vertices and one at each of the two ends.
- The end of a line = a terminal carbon (CH3 if it has three implied hydrogens).
- A vertex between two lines = an internal carbon.
- A double line = a double bond; a triple line = a triple bond, still between two carbons.
Rule 2: hydrogens on carbon are implied
Carbon almost always forms four bonds. In a skeletal structure you count the bonds you can see going to a carbon, then mentally add enough hydrogens to reach four. That is where the hydrogens are "hiding" — they are implied, not drawn.
- A carbon with 1 visible bond has 3 implied H (it's a CH3 group).
- A carbon with 2 visible bonds has 2 implied H (a CH2).
- A carbon with 3 visible bonds has 1 implied H (a CH).
- A carbon with 4 visible bonds has 0 implied H.
Remember that a double bond counts as two of a carbon's four bonds. So a carbon at the end of a C=C double bond (one double bond, nothing else drawn) still needs 2 hydrogens: =CH2.
Heteroatoms are always drawn
Any atom that is not carbon or an implied hydrogen must be written explicitly: O, N, S, the halogens (F, Cl, Br, I), and so on. These are called heteroatoms. Unlike carbon, hydrogens attached to a heteroatom are shown — you write the O–H of an alcohol as "OH" and the N–H of an amine as "NH2." This is important because those O–H and N–H hydrogens are the ones that participate in hydrogen bonding, acidity, and many reactions, so you can't afford to leave them ambiguous.
For heteroatoms, you fill in hydrogens to complete their normal valence: oxygen wants 2 bonds, nitrogen wants 3 (neutral), and the halogens want 1. So an oxygen drawn with a single bond to a carbon and no other bond gets one hydrogen written in as OH.
Converting between representations
You'll constantly translate between three formats. Being fluent in all three is one of the first real skills in orgo:
- Lewis / full structural: every atom and bond shown explicitly (great for tracking electrons, slow to draw).
- Condensed: written on one line, like CH3CH2CH2OH, grouping each carbon with its hydrogens.
- Bond-line: the skeletal zig-zag described here (fastest, cleanest for mechanisms).
To go from condensed to bond-line, place a vertex for each carbon and drop the hydrogens on carbon; keep the heteroatoms. CH3CH2OH (ethanol) becomes a two-vertex zig-zag ending in "OH." To go the other way, count vertices for carbons, add implied hydrogens back to reach four bonds each, and keep the heteroatom H's as written.
Why chemists prefer them and common mistakes
Skeletal structures are fast to draw, uncluttered, and make functional groups and reaction sites jump out — which is exactly what you want when pushing curved arrows through a mechanism. That speed is why nearly every textbook mechanism uses them.
Watch out for the classic beginner errors:
- Forgetting a carbon has four bonds and drawing too many or too few implied H.
- Leaving off hydrogens on a heteroatom — an O with one bond is an OH, not a bare O.
- Miscounting a double bond as one bond instead of two when totaling a carbon's bonds.
- Putting a C at the end of every line but forgetting the terminal CH3 is still a carbon with three hydrogens.
Once these rules are automatic, reading skeletal structures becomes second nature and you'll never want to write out full Lewis structures again.
Draw this on the whiteboard
Open the OChem Board whiteboard — benzene rings, curved arrows, wedge/dash bonds and a clickable periodic table built in. No account needed.