Final answer:
A hydrocarbon with a double bond between carbon atoms is called an alkene, which is an unsaturated hydrocarbon. Alkenes have a general formula of CnH2n for molecules with one double bond and differ in their geometrical structure from saturated hydrocarbons, which only have single bonds.
Step-by-step explanation:
A hydrocarbon that has a double bond between carbon atoms is known as an alkene. Alkenes are a type of unsaturated hydrocarbon, meaning they do not contain the maximum number of hydrogen atoms possible because they have one or more double bonds instead of single bonds. A saturated hydrocarbon molecule, on the other hand, contains only single bonds between carbon atoms, which means each carbon is bonded to four other atoms (hydrogen or carbon), and are called alkanes.
The general formula for alkenes with one double bond (assuming non-cyclic structures) is CnH2n, reflecting that these molecules have two fewer hydrogen atoms than alkanes. For example, the simplest alkene, ethene (also known as ethylene), has the formula C2H4. Molecules with multiple double bonds are also common, such as 1,3-butadiene, which has two double bonds in a four-carbon chain.
Double bonds introduce a different geometry around the carbon atoms, causing differences in molecular shape and properties when compared to saturated hydrocarbons. Therefore, the physical and chemical behaviors of alkenes can significantly differ from those of alkanes.