Which of the following halides forms salt like KHX \(_{2}\) (where \(\mathrm{X}\) is a halogen atom \()\) ? (a) HI (b) \(\mathrm{HBr}\) (c) \(\mathrm{HCl}\) (d) HF

When it comes to the chemistry of the halogens, 'halide reactivity' encompasses the ways in which halogens engage in chemical processes due to their distinctive characteristics. Reactivity among halides can vary significantly between fluoride, chloride, bromide, and iodide. A primary determinant of this reactivity is the electron affinity and the relative size of the halogen atoms.

For instance, fluoride (F-) is highly reactive due to its small size and high electron affinity, making it less likely to participate in the formation of polyhalide ions, which are molecules that contain more than one halogen atom. Iodide (I-), on the other hand, has a lower electron affinity and is larger in size, which makes it more conducive to forming these complex ions. This difference in reactivity is crucial when identifying the correct halide capable of forming compounds like KHX2.

Polyhalide ions represent an intriguing aspect of halogen chemistry. They consist of complex ions with more than one halogen atom, typically forming when a halogen molecule reacts with a halide ion. In the context of the exercise, we are examining the propensity of different halides to form salts with the general formula KHX2.

The possibility of such salts crystallizing relies heavily on the formation of stable polyhalide ions. Iodine is particularly known for its ability to stabilize extra halide ions, resulting in ions like I3-. Such a phenomenon is due to iodine's larger atomic radius allowing for easier accommodation of additional negatively charged ions. In iodine's case, when K+ and I- come together in the presence of molecular iodine (I2), the I3- ion can form, subsequently enabling the creation of the salt KHI2.

The group of elements known as 'Halogens' occupy Group 17 of the periodic table and include fluorine, chlorine, bromine, and iodine. They are notorious for their high reactivity, which derives from their quest to achieve a complete outer electron shell.

In-depth knowledge of the distinctive behaviors and chemical properties of halogens is fundamental for understanding reactions like polyhalide formation. Iodine's tendency to form polyhalide ions, as demonstrated in the exercise solution where HI forms KHI2, is an example of the unique characteristics within this group. Halogens often contribute significantly to the nature of organic and inorganic chemistry, as seen in their roles in disinfection, medication, and as catalysts in different types of reactions, making them an essential study topic for any chemistry student.