Question

The correct order of ionic radii is: (a) \(\left.\mathrm{I}^{+}>\mathrm{l}^{-}\right\rangle \mathrm{I}\) (b) \(\mathrm{I}>\mathrm{I}^{+}>\mathrm{I}^{-}\) (c) \(\mathrm{I}^{+}>\mathrm{I}>\mathrm{I}^{-}\) (d) \(\mathrm{I}^{-}>\mathrm{I}>\mathrm{I}^{+}\)

Short answer

The correct order of ionic radii is (d): \(\mathrm{I}^{-} > \mathrm{I} > \mathrm{I}^{+}\).

Step-by-step solution

Understanding Ionic Radii

Ionic radius is the measure of an atom's ion in a crystal lattice. Cations (positively charged ions) are typically smaller than their neutral atom counterparts, while anions (negatively charged ions) tend to be larger.

Analyzing Ions Given

Given iodine atoms: \(\mathrm{I}\), \(\mathrm{I}^{+}\) (a cation), and \(\mathrm{I}^{-}\) (an anion). Since \(\mathrm{I}^{+}\) loses an electron, it is smaller than \(\mathrm{I}\), and \(\mathrm{I}^{-}\), gains an electron, which makes it larger than \(\mathrm{I}\).

Comparing Ionic Sizes

Considering the electron configuration, \(\mathrm{I}^{-}\) has more electrons, leading to increased electron-electron repulsion and a larger size. \(\mathrm{I}^{+}\) has fewer electrons and less repulsion, making it smaller. Thus the order in size: \(\mathrm{I}^{-} > \mathrm{I} > \mathrm{I}^{+}\).

Selecting the Correct Option

Based on our analysis, the option that reflects the correct order of ionic radii is option (d): \(\mathrm{I}^{-} > \mathrm{I} > \mathrm{I}^{+}\).

Key concepts

Iodine Ions

Iodine is an element commonly found in compounds such as iodides. In the context of ions, iodine can exist in multiple ionic forms, specifically as an anion ( I^{-} ) or a cation ( I^{+} ). An iodine anion gains an extra electron, making it negatively charged. This addition increases its electron cloud, causing it to expand. This increased repulsion between electrons means that I^{-} is larger than a neutral iodine atom. In contrast, when iodine becomes a cation, it loses an electron, resulting in a positively charged ion, I^{+} . This loss means there's fewer electron-electron repulsions, and the electron cloud can contract, making I^{+} smaller. Hence, each form of iodine ion has a specific size based on the number of electrons. This variation in ionic size is crucial for understanding why I^{-} is larger than neutral iodine ( I ), and neutral iodine is larger than I^{+} .

Electron Configuration

Electron configuration refers to the distribution of electrons in an atom or ion. In neutral iodine, represented simply as I , electrons occupy specific energy levels or shells in a defined order. As electrons fill the shells from lower to higher energy, the overall arrangement determines many of the chemical properties of the element. For iodine, its neutral configuration is [Kr]4d^{10}5s^{2}5p^{5} . When iodine forms an anion to become I^{-} , an additional electron is added, filling the 5p orbital, resulting in the configuration [Kr]4d^{10}5s^{2}5p^{6} . This added electron introduces more electron-electron repulsion, expanding the ion's size. When forming a cation, iodine loses an electron, which is typically removed from the outermost orbital, often the 5p orbital. This results in a positive ion with the configuration [Kr]4d^{10}5s^{2}5p^{4} . The electron loss reduces repulsion forces, causing the ionic radius to decrease. Understanding these configurations can help predict the chemical behavior of iodine ions.

Cations and Anions

Cations and anions are ions that carry a net charge, either positive or negative, respectively. Cations are produced when an atom loses one or more electrons, resulting in a positively charged ion. This loss in electrons reduces the size of an atom, as seen in I^{+} , where reduced repulsion between fewer electrons leads to a smaller ionic radius. Anions, on the other hand, form when an atom gains one or more electrons, creating a negatively charged ion. With additional electrons, as in I^{-} , the resultant increase in electron-electron repulsion leads to a larger ionic radius. These changes in size are directly linked to the number of electrons added or removed. Overall, understanding these changes helps in predicting the physical and chemical properties of ions as well as their interactions with other elements in forming compounds.