Why is ${ I }_{ 2 }$ less reactive than ICl?

Hint: You can explain this fact, that ICl is more reactive than ${ I }_{ 2 }$, in the same way as you explain why the reactivity of the diatomic halogen molecules decreases as the group descends. Now try to answer this question accordingly.

Complete step by step solution:
> We should know that ICl is more reactive than diatomic ${ I }_{ 2 }$ as the atomic radius of the Cl atom is smaller than that of the I atom, meaning that the first electron affinity of the Cl atom is greater than I atom.
> When Cl gains an electron it is being added to an orbital at a lower energy level thus the electron loses more energy so has lower intrinsic energy and is closer to the nucleus meaning that there is a stronger electrostatic force of attraction between the outer electron and the nucleus which requires more energy to break down.
- This means that the product formed (the ${ Cl }^{ - }$ ion) is at a lower energy level compared to the ${ I }^{ - }$ ion formed in the first electron affinity of I and therefore more stable.
- This whole concept combined with the fact that the overall reaction is more exothermic, meaning that the reaction is more thermodynamically feasible. This explains why ICl is more reactive than ${ I }_{ 2 }$.
- In simple words, we can say that the interhalogens are more reactive than halogens (except for Fluorine) because A-X bonds in interhalogens are weaker than the X-X bonds in dihalogen molecules.
Therefore, we described the reason for less reactivity of ${ I }_{ 2 }$ compared to ICl.

Note: This is also a fact that Iodine is the least reactive of the halogens. This is the most electropositive among halogens, meaning it has the tendency to lose electrons and form positive ions during chemical reactions.
The reaction of interhalogens are similar to halogens. The hydrolysis of interhalogen compounds gives halogen acid and oxy-acid.