Reactivity refers to the ability of a substance to undergo chemical reactions. Several factors influence reactivity, including:

Bond strength: Weaker bonds are more easily broken, making molecules more reactive. In the case of C2H4 and C2H6, the C=C double bond in C2H4 is weaker than the C-C single bonds in C2H6. This means that the C=C bond is more likely to break, making C2H4 more reactive.

Number of reactive sites: The more reactive sites a molecule has, the more likely it is to react. C2H4 has two reactive sites (the two carbon atoms of the double bond), while C2H6 has only one (the carbon atom of the single bond). This means that C2H4 has more opportunities to react, making it more reactive.

Stability: Less stable molecules are more likely to react to achieve a more stable state. C2H4 is less stable than C2H6 because the double bond is a higher energy state than the single bond. This means that C2H4 is more likely to react to achieve the more stable single bond configuration of C2H6.

Overall, the weaker bond strength, greater number of reactive sites, and lower stability of C2H4 compared to C2H6 make C2H4 more reactive. This is reflected in the fact that C2H4 readily undergoes a wide variety of reactions, including addition, polymerization, and combustion, while C2H6 is relatively unreactive.