You must find all possible unique structures of a molecule having a given formula. From past experience, chemists know that there are certain arrangements of atoms that are much more probable than others. These are represented by solid objects with connectors in molecular model kits. Hydrogen can form one bond. Oxygen is represented by a red object.
Oxygen forms two bonds.
Oxygen can form one double bond or two single bonds. Carbon is represented by a black object. Carbon forms four bonds. Chlorine is represented by a green object. Chlorine in organic compounds can form one bond.