The halogen which is most reactive in the halogenation of alkanes under sunlight is Chlorine.

Bromination of alkanes occurs by a similar mechanism, but is slower and more selective because a bromine atom is a less reactive hydrogen abstraction agent than a chlorine atom, as reflected by the higher bond energy of H-Cl than H-Br.

Chlorine is the most reactive halogen in the halogenation of alkanes under sunlight.

Explanation:
When alkanes react with halogens in the presence of sunlight or heat, a substitution reaction called halogenation occurs. In this reaction, one or more hydrogen atoms in the alkane are replaced by halogen atoms.

The reactivity of halogens in the halogenation of alkanes depends on their ability to undergo homolytic cleavage, which means breaking the halogen-halogen bond to form two halogen radicals. These halogen radicals then react with the alkane to form a halogenated alkane.

The reactivity of halogens in homolytic cleavage is influenced by two factors: bond dissociation energy and bond length.

Bond dissociation energy:
The bond dissociation energy is the energy required to break a bond. The lower the bond dissociation energy, the easier it is to break the bond and form radicals. In the case of halogens, the bond dissociation energy decreases as we move down the group. This means that the halogen-halogen bond becomes weaker and easier to break.

Bond length:
The bond length is the distance between the nuclei of two bonded atoms. As we move down the halogen group, the bond length increases. This is because the number of electron shells increases, resulting in a larger atomic radius and longer bond length.

Explanation of the answer:
Chlorine is the most reactive halogen in the halogenation of alkanes under sunlight because it has the lowest bond dissociation energy and a relatively short bond length compared to other halogens.

Bond dissociation energy:
The bond dissociation energy of chlorine is lower than that of bromine, iodine, and fluorine. This means that the chlorine-chlorine bond is easier to break, resulting in the formation of chlorine radicals. These chlorine radicals are highly reactive and readily react with alkanes.

Bond length:
The bond length of chlorine is shorter than that of bromine, iodine, and fluorine. This shorter bond length allows for better overlap of the chlorine orbital with the sigma bond in the alkane, facilitating the homolytic cleavage of the chlorine-chlorine bond and the subsequent reaction with the alkane.

Therefore, due to its low bond dissociation energy and relatively short bond length, chlorine is the most reactive halogen in the halogenation of alkanes under sunlight.