Problem 53 - A Complete Resource Book in Chemistry for JEE Main

Question

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

Step-by-Step Solution

Answer

The correct order is (d): I^- > I > I^+.

Step 1: Understanding Ionic Radii

Ionic radius refers to the size of an ion. Positive ions (cations) are smaller than their neutral atoms because they lose electrons, which results in less electron repulsion and a smaller radius. Negative ions (anions) are larger than their neutral atoms because they gain electrons, increasing electron repulsion and the ionic size.

Step 2: Determining the Sizes of Ions

For iodine (I), I^- (iodide ion) is larger than I as it gains an electron, increasing repulsion and size. I^+ is smaller than neutral I as it loses an electron, reducing repulsion and size.

Step 3: Evaluating Each Option

- Option (a): I^+ > I > I^- is incorrect because I^+ should be smaller than I. - Option (b): I > I^+ > I^- is incorrect because it implies I is larger than both ions. - Option (c): I^+ > I > I^- is not logically possible as I should be larger than I^+. - Option (d): I^- > I > I^+ correctly reflects that anions are larger than neutral atoms, which are larger than cations.

Step 4: Conclusion

Based on the principle that cations are smaller and anions are larger, the only correct order of ionic radii is reflected in option (d): I^- > I > I^+. This follows the understood trends in ionic radii.

Key Concepts

Cations and Anions

In chemistry, the terms 'cations' and 'anions' refer to ions with opposite charges, which significantly affects their sizes. Cations are positively charged ions formed when an atom loses one or more electrons. This loss results in a greater positive charge of protons compared to the negative charge of electrons.

Because cations have less electrons than their neutral atoms, they experience a reduction in electron-electron repulsion. This causes the cation's electrons to be pulled closer to the nucleus, making the cation smaller in size than its corresponding neutral atom.

On the other hand, anions are negatively charged ions. They are created when an atom gains additional electrons, which increases the total negative charge. The new electrons enhance electron-electron repulsion, causing the electron cloud to expand. This makes anions larger than the neutral atoms from which they originate.

Electron Repulsion

Electron repulsion is a fundamental concept in chemistry that helps explain the differences in sizes between cations, neutral atoms, and anions. When electrons are added to an atom, as in the case of anions, increased electron-electron repulsion occurs within the electron cloud.

This repulsion results in a greater spread of electrons around the nucleus, thus increasing the ionic radius. Conversely, when electrons are removed to form cations, electron-electron repulsion decreases. The effective nuclear charge experienced by the remaining electrons becomes stronger and pulls the electron cloud closer to the nucleus, reducing the ionic radius.

Understanding electron repulsion thus allows us to predict and rationalize the changes in ionic sizes as ions gain or lose electrons. This principle is essential for recognizing why cations are smaller and anions are larger than their neutral atoms.

Trends in Periodic Table

The periodic table showcases several trends in elemental properties, including ionic radii, which reflects patterns of cation and anion sizes. Across a period (from left to right), cations generally become smaller. This is due to an increasing number of protons exerting a stronger pull on the electron cloud, despite losing electrons.

Down a group (from top to bottom), the ionic radii increase. This increase occurs because additional electron shells are added, which outweighs the increased nuclear charge due to more protons. Additional shells keep electrons further from the nucleus.

For anions, the trend across a period still results in gradually decreasing sizes, although anions remain larger than cations. These trends provide valuable insights into the chemical behavior of elements, guiding predictions about reactivity and stability based on ionic sizes.