Bromine (Br) and Iodine (I) are non-metals belonging to the Halogen family (Group 17) of the periodic table. Both elements seek to gain a single electron to achieve a stable outer shell. The element that can capture that electron more easily is considered more reactive. Bromine is more reactive than Iodine, a difference explained by their underlying atomic structures.
Understanding Reactivity in the Halogen Family
The Halogen family includes Fluorine, Chlorine, Bromine, Iodine, and Astatine. These elements exhibit a clear pattern where reactivity decreases steadily as you move down the group. Fluorine is the most reactive halogen, followed by Chlorine, then Bromine, and Iodine is the least reactive of the common halogens.
Halogens are highly reactive because they possess seven electrons in their outermost shell, making them one electron short of a stable shell. The ease with which a halogen atom can attract and gain this single electron determines its overall reactivity. For non-metals, a greater ability to accept an electron means higher reactivity.
The trend of decreasing reactivity is a direct consequence of the increasing size of the atoms as you descend the group. This size difference between Bromine and Iodine dictates their relative chemical behavior. Bromine is positioned above Iodine, making it inherently more chemically active.
Key Factors Governing Bromine and Iodine Reactivity
The difference in reactivity is linked to their atomic structures. Iodine atoms are significantly larger than Bromine atoms because Iodine possesses an extra electron shell. This size difference introduces two effects that weaken the Iodine atom’s ability to attract an incoming electron.
Atomic Radius
The increased atomic radius in Iodine means the nucleus is farther away from the valence shell. Since the nucleus carries the positive charge that attracts electrons, this greater distance results in a weaker pull on a new electron. Conversely, the nucleus of the smaller Bromine atom is closer to its outer shell, allowing it to exert a stronger attractive force.
Shielding Effect
The larger Iodine atom has a greater number of inner electron shells, leading to a stronger “shielding effect.” These inner electrons repel the outer and incoming electron, effectively shielding the positive pull of the nucleus. This stronger shielding further diminishes the net attractive force on the electron Iodine needs to gain.
These two factors result in Iodine having a lower electron affinity compared to Bromine. Electron affinity measures the energy change when an atom gains an electron; a higher value indicates a stronger desire to accept an electron. Since Bromine’s nucleus attracts the incoming electron more effectively, it has a higher electron affinity and is more chemically reactive.
Observing the Reactivity Difference in Practice
The difference in reactivity between Bromine and Iodine is demonstrated through displacement reactions. A more reactive halogen can displace a less reactive halogen from a solution of its salt. This displacement occurs because the more reactive element has a greater attraction for electrons.
For example, when Bromine is added to a solution containing Iodide ions (e.g., potassium iodide), the Bromine takes the electron from the Iodide. This forms Bromide ions and elemental Iodine, often causing the solution to turn brown. This reaction confirms that Bromine is the stronger chemical agent, successfully displacing the Iodide.
Conversely, if Iodine is added to a solution containing Bromide ions, no reaction will occur. Iodine is not reactive enough to displace Bromine from its compound because it has a weaker pull for the electron. The ability of Bromine to displace Iodine, but not the reverse, serves as experimental proof of the reactivity trend.
This difference also relates to the elements’ strength as oxidizing agents. The halogen that most readily accepts an electron is considered the strongest oxidizing agent. Since Bromine has a greater tendency to gain an electron than Iodine, it is a stronger oxidizing agent, readily causing the oxidation of Iodide ions during a displacement reaction.