Organic chemistry is the chemistry of carbon compounds, and there are millions of them. They become manageable once you sort them into families that share a functional group and a general formula, and learn to write each molecule in the four ways examiners ask for.
A molecular formula gives the number and type of each atom (C2H6O); a structural formula shows how the atoms are arranged without drawing bonds (CH3CH2OH); a displayed formula shows every atom and every bond. A functional group is the atom or group of atoms that decides the chemical properties of a compound: C=C in alkenes, −OH in alcohols, −COOH in carboxylic acids. A homologous series is a family of compounds with the same general formula and same functional group, so they have similar chemical properties and a trend in physical properties; neighbouring members differ by CH2. A saturated compound has only C−C single bonds; an unsaturated compound contains at least one C=C double (or C≡C triple) bond.
Build a molecule in the simulator and switch between its molecular, structural and displayed formulae. Notice that the displayed formula is the only one that shows every single bond.
| Type | What it shows | Propane | Ethanoic acid |
|---|---|---|---|
| molecular | number of each type of atom | C3H8 | C2H4O2 |
| structural | arrangement of atoms, no bonds drawn | CH3CH2CH3 | CH3COOH |
| displayed | every atom and every bond as a line | draw each C with four bonds, each H with one, each O with two; C=O is two lines | |
| general | formula for the whole series | CnH2n+2 | CnH2n+1COOH |
Check a displayed formula by counting bonds: carbon 4, oxygen 2, hydrogen 1. A carbon with only three lines is the commonest drawing error.
Four quick checks. Each correct answer earns XP and records this skill on your profile.
What does a displayed formula show?
Which is the functional group of the carboxylic acids?
What is the general formula of the alkanes?
Which compound is unsaturated?
Members of a homologous series behave alike chemically because they share a functional group. Their physical properties change gradually as the chain gets longer.
A homologous series has similar chemical properties but a trend in physical properties: do not swap the two words. "Saturated" means only single carbon-carbon bonds, not "contains as much hydrogen as possible dissolved in it". And the −OH in an alcohol is a covalently bonded group, not a hydroxide ion: alcohols are not alkalis.
Unlocks once the four checks above are done. Worth more XP, written to IGCSE Paper 2 and Paper 4 standard.
Which statement about the members of a homologous series is correct?
An alkene has 5 carbon atoms. What is its molecular formula?
Which is the structural formula of propan-1-ol?
Consecutive members of a homologous series differ by:
This skill is now recorded on your profile. Keep the chain going.