Alkenes
In alkanes there are only single bonds between carbon atoms. There are also hydrocarbons with double bonds between carbon atoms.
Alkenes
Hydrocarbons with one or more double bonds between carbon atoms are classed as alkenes. The simplest alkene is ethene, with the molecular formula C2H4.
The two lines between the carbon atoms are the double bond. Because of the double bond between the carbon atoms, alkenes are more reactive than alkanes.
Ethane, an alkane
C2H6
A single bond between the carbons; each carbon holds three hydrogens.
Fairly unreactive — mostly just burns.
Ethene, an alkene
C2H4
A double bond between the carbons; each carbon holds two hydrogens.
More reactive — the double bond can open.
Why the double bond matters
Count the bonds on each carbon of ethene: two to hydrogens and two in the double bond — four, as always. A double bond is two shared pairs, and the second pair is held less firmly than the first. It can open up, leaving each carbon with a free bond that can join to something else, while the first bond still holds the two carbons together. That is why alkenes react where alkanes do not, and it is the whole key to the polymers of the next section: open the double bonds of many ethene molecules and they can link into a chain.
The summary also mentions hydrocarbons with triple bonds between carbon atoms, the alkynes. They are named only; this chapter does not study them.
Alkenes have a C=C double bond
The simplest alkene is ethene, C2H4. Its double bond makes alkenes more reactive than alkanes.