Explanation:

When a cation and an anion combine in a crystal, they form an ionic compound. Ionic compounds are made up of positively charged ions (cations) and negatively charged ions (anions) held together by strong electrostatic forces.

Key Points:

- Cations are formed when an atom loses one or more electrons and becomes positively charged. Anions are formed when an atom gains one or more electrons and becomes negatively charged.
- In an ionic compound, the cations and anions are arranged in a regular repeating pattern called a crystal lattice.
- The strong electrostatic forces between the cations and anions hold the crystal lattice together.
- Ionic compounds have high melting and boiling points because a large amount of energy is required to break the strong electrostatic forces holding the ions together.
- Ionic compounds are typically solid at room temperature and are often crystalline in nature.
- Examples of ionic compounds include sodium chloride (NaCl), calcium carbonate (CaCO3), and magnesium oxide (MgO).

Properties of Ionic Compounds:

- Ionic compounds are good conductors of electricity when they are dissolved in water or melted because the ions are free to move and carry electric charge.
- Ionic compounds are typically brittle because the crystal lattice can easily break along planes of ions of the same charge.
- Ionic compounds have a high degree of solubility in polar solvents such as water.

Conclusion:

In conclusion, when a cation and an anion combine in a crystal lattice, they form an ionic compound. Ionic compounds have specific properties and characteristics that distinguish them from other types of compounds.