| Formula Type | Shows | Ethanol Example |
|---|---|---|
| Empirical | Simplest whole-number ratio | C₂H₆O |
| Molecular | Actual number of each atom | C₂H₆O |
| Full structural | Every bond drawn explicitly | H-C-C-O-H (with all H) |
| Condensed | Groups written sequentially | CH₃CH₂OH |
| Skeletal | C-C backbone as zig-zag; H on C omitted | Line with -OH at end |
Examiner Trap
Full Structural Formula: Show Every Single Bond
In a full structural formula, you must show every single bond explicitly. Writing -O-H as -OH or -C(=O)OH as -COOH is technically condensed and loses marks when a full structural formula is requested.
Skeletal Formulas
Implicit Hydrogens in Skeletal Structures
When interpreting skeletal structures, remember each vertex/end is a carbon atom with enough implicit C–H bonds to give carbon 4 total bonds:
- End of a chain (1 bond shown) = -CH₃ (3 implicit H's)
- Bend in a chain (2 bonds shown) = -CH₂- (2 implicit H's)
- Branch point (3 bonds shown) = >CH- (1 implicit H)
- Crossroad (4 bonds shown) = >C< (0 implicit H's)
Connectivity Trap
Correct Bonding for -COOH and -CHO Groups
Always draw functional groups with correct atom connectivity:
- Draw -COOH connected through Carbon:
-C(=O)OH(not-O-C(=O)H) - Draw -CHO connected through Carbon:
-C(=O)H(not-C-O-H) - Draw -OH connected through Oxygen:
-O-H(not-H-O)
AQA GCSE & IB Chemistry
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