4. Both the alkene C4H8 and the alkyne C4H6 have rigid bond structures. However, C4H8 can form three isomers, whereas C4H6 can form only two isomers. Why is this so?

Both alkene C4H8 and alkyne C4H6 have rigid bond structures. C4H8 can form three isomers while C4H6 can only form two isomers. Isomers are defined as two or more compounds having the same formula but with different molecular arrangements. The alkene contains 1 double bond which connects 2 carbons, so the other 4 hydrogen atoms can be rearranged. The alkyne only has 2 extra hydrogen atoms.