Final answer:
In the ethene molecule (C₂H₄), there are two carbon atoms.
Step-by-step explanation:
The molecular formula C₂H₄ represents ethene, a hydrocarbon. In this formula, the subscript '2' accompanying 'C' signifies the presence of two carbon atoms in each molecule of ethene. This compound is a simple hydrocarbon commonly known for its unsaturated nature due to the presence of a double bond between the carbon atoms. Understanding molecular formulas and their subscripts is crucial for accurately describing the composition of chemical compounds. Therefore, in the case of C₂H₄, the number of carbon atoms is two.