What are Ionic Compounds?
Ionic compounds are chemical compounds that are made up of differently charged ions (which are held together by ionic bonds). The positively charged ions are commonly referred to as cations whereas those that are negatively charged are called anions. Ions can be monoatomic (such as the potassium cation, K+) or polyatomic in nature. The ammonium cation (NH4+) is a good example of a polyatomic cation. Ammonium nitrate is an ionic compound formed by a polyatomic cation (NH4+) and a polyatomic anion (NO3–).
The individual ions in ionic compounds are generally surrounded by several neighboring ions, resulting in an elaborate, three-dimensional crystal structure. The ionic compounds that contain at least one hydrogen ion (H+) are considered to be acids. Similarly, those holding a hydroxide ion (OH–), or an oxide ion (O2-) are said to be basic in nature. The neutralization reactions between acids and bases result in the formation of a special class of ionic compounds known as salts.
Properties of Ionic Compounds
Generally, the compounds held together by an ionic bond have high melting points and boiling points. In their solid states, these compounds easily crumble into a powder. When a comparison is made between their solid and molten state, a huge contrast in the electrical conductivity of ionic compounds can be observed. Ionic compounds are electrically insulating in their solid states because the ions are tightly packed in the crystalline structure. When the compound is melted, these ions become mobile, resulting in increased conductivity.
Several ionic crystals also contain water of crystallization. The properties and crystal structures of these hydrates can vary. For example, nine crystalline forms of zinc chloride and its hydrates are known. The four polymorphs of this compound have tetrahedral geometries.
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