The electron affinity of an atom refers to the energy change that occurs when an atom in the gas phase gains an electron to form a negatively charged ion. In the case of bromine (Br) and iodine (I), the electron affinity of bromine is greater than that of iodine.
There are several factors that contribute to this difference:
Atomic size: Bromine has a smaller atomic size compared to iodine. As the atomic size increases, the outermost electrons are farther from the nucleus, resulting in weaker attractive forces between the nucleus and incoming electrons. In the case of iodine, its larger size makes it more difficult for the incoming electron to be effectively attracted and captured by the nucleus, leading to a lower electron affinity.
Electron shielding: As the atomic size increases, there are more electron shells between the nucleus and the outermost electrons. These inner shells act as a shield, reducing the effective nuclear charge experienced by the outermost electrons. In iodine, the increased number of electron shells decreases the effective attraction between the nucleus and the incoming electron, resulting in a lower electron affinity compared to bromine.
Electron-electron repulsion: In larger atoms like iodine, there is greater electron-electron repulsion due to the increased number of electrons present. This repulsion makes it energetically less favorable for an additional electron to be added to the atom, resulting in a lower electron affinity.
It's important to note that while these factors generally contribute to the difference in electron affinity between bromine and iodine, electron affinity values can also be influenced by other factors such as electronic configurations and specific orbital interactions. The observed electron affinity values are determined experimentally and may deviate from the expected trends in certain cases due to these additional factors.