Alkane's are formed from carbon and hydrogen atoms, a compound known as a hydrocarbon, in single bonds. Alkenes are also hydrocarbons, but contain a double bond. In both compounds, Carbon must always bond to 4 other atoms meaning it will have 4 lines coming off it, Hydrogen only needs one. here you'd include a drawing to show the difference between the two As you can see, the number of Hydrogen atoms in an alkane, is double the number of Carbon atoms + 2 extra hydrogens. whereas alkenes don't have the 2 extra hydrogen atoms due to the double bond meaning the carbons still have 4 bonds. This can be expressed as: Alkanes CnH2n+2 and Alkenes CnH2n. A way I remember this, is that alkenes have the double bond because the Carbons hold on to each other extra tight! if student is unsure on how to name either, explain the "Monkeys Eat Purple Bananas, Penguins Hate Heavy Objects" way to remember them in order