Alkanes
Alkanes are saturated hydrocarbons (only single C-C bonds) with the general formula . They're named with a prefix showing the number of carbons (meth-, eth-, prop-, but-...) plus the ending "-ane".
Example 1: Give the molecular formula of propane, and state how many hydrogen atoms it has.
Propane has 3 carbons, so using : , meaning 8 hydrogen atoms.
Alkenes
Alkenes are unsaturated hydrocarbons containing at least one C=C double bond, with the general formula , named with the ending "-ene". Because of the double bond, alkenes are more reactive than alkanes and decolourise bromine water, while alkanes do not -- this is the standard chemical test used to tell them apart.
Example 2: A hydrocarbon decolourises bromine water. What does this tell you about the compound, and give its likely general formula?
Decolourising bromine water indicates the presence of a C=C double bond, so the compound is unsaturated -- an alkene, with general formula .
Functional groups
A functional group determines a molecule's typical reactions regardless of how long its carbon chain is. Alcohols contain the group and are named with the ending "-ol" (e.g. ethanol, ). Carboxylic acids contain the group and are named with the ending "-oic acid" (e.g. ethanoic acid, , the acid in vinegar).
Isomerism
Structural isomers are compounds with the same molecular formula but different arrangements of atoms, giving them different properties. For example, both butane and methylpropane share the formula , but butane has a straight chain of 4 carbons while methylpropane has a branched chain -- different structures, same formula.