Define ionic radius. How does the size of an atom change when it is converted to (a) an anion and (b) a cation?

Understanding the size of anions and cations is fundamental to grasping the concepts of ionic compounds. An important aspect to consider is how the size of an atom changes upon gaining or losing electrons.

When an atom gains electrons, it becomes an anion, which typically results in the ionic radius increasing. Why does this happen? The atom now has more electrons than protons, leading to a decreased attraction per electron. This means there is less hold by the nucleus over the overall larger electron cloud, which spreads out due to increased electron-electron repulsion. The result is a physically larger ion.

Conversely, when an atom loses electrons and becomes a cation, the ionic radius decreases. This is because there are now fewer electrons in the cloud, and the positive charge of the protons has a stronger pull on the remaining electrons, causing the electron cloud to contract. The overall effect is a smaller ion compared to the neutral atom from which it was formed.

This difference in size has important consequences in the formation and properties of ionic compounds. A simple approach to remember is that cations shrink in size, relative to the original atom, and anions expand.

The phenomenon of electron-electron repulsion is at the heart of many observations related to atomic and molecular structures. Electrons are negatively charged particles, and like charges repel each other according to electrostatic principles. In the context of anions and cations, this concept is crucial.

When an atom becomes an anion, the additional electrons increase the negative charge density around the nucleus. These extra electrons repel each other due to their like charges. The repulsion forces the electron cloud to expand outward, resulting in an increase in ionic radius.

In the case of cations, the loss of electrons means there are fewer electron-electron repulsions within the electron cloud, allowing the positively charged nucleus to pull remaining electrons closer, thus reducing the ionic radius.

It’s beneficial to visualize electron-electron repulsion as a balancing act between the attractive force of the nucleus and the repelling forces among the electrons. When there are more repulsion factors at play, the atom's size increases and vice versa.

The crystal lattice is a three-dimensional arrangement of ions in a crystal. Every point in the lattice is occupied by either an anion or cation, forming a repeating pattern that extends throughout the solid.

The ionic radius plays a significant role in the structure of a crystal lattice. It determines the distances between neighboring ions, thereby affecting the energy stability and the physical properties of the compound, such as melting point and hardness.

In a crystal lattice, the ions are packed together in a way that optimizes attractive forces and minimizes repulsive forces to achieve stability. This packing is also influenced by the size of the ions; smaller cations often occupy interstices (spaces) between larger anions. The efficient packing of these ions determines the overall stability of the crystal lattice structure.

It is the interplay of ionic sizes and the forces between them that determine the unique lattice structures of different ionic compounds. Hence, the understanding of ion sizes and the concept of electron-electron repulsion provides insight into predicting and explaining the arrangement of ions in a crystal lattice.