An ionic compound is a chemical compound formed when atoms transfer electrons between each other. It typically happens between a metal and a nonmetal. Metals tend to lose electrons and become positively charged ions, known as cations, while nonmetals gain electrons to form negatively charged ions called anions. ^[1-4]

Once these oppositely charged ions form, they are held together by strong electrostatic forces. This force of attraction is known as an ionic bond, a strong chemical bond that holds the ions together in a fixed ratio and forms a stable compound with a defined structure.

Properties ^[1-4]

1. High Melting and Boiling Points

Ionic compounds have high melting and boiling points because the strong electrostatic attraction between ions requires a lot of energy to break. This is why most ionic compounds are solid at room temperature.

Example: Sodium chloride (NaCl) melts at around 801°C.

2. Solubility

Most ionic compounds dissolve easily in polar solvents, such as water, and are not soluble in nonpolar solvents, like organic solvents (benzene, toluene, etc.). Water has positively and negatively charged ends that can attract the ions in the compound and pull them apart.

Example: Calcium chloride (CaCl₂) dissolves into Ca²⁺ and Cl^– ions.

3. Electrical Conductivity

In solid form, ionic compounds do not conduct electricity because the ions are locked in place. However, when melted or dissolved in water, the ions are free to move and carry electric current, making them excellent conductors in those states.

Example: Molten or aqueous sodium chloride (NaCl) can conduct electricity.

4. Crystalline Structure

Ionic compounds form a crystal lattice, a repeating 3D pattern of ions. This structure makes them hard and stable and gives them a regular shape.

Example: NaCl forms a face-centered cubic (FCC) crystal lattice, where each Na⁺ ion is surrounded by six C^l- ions and vice versa.

5. Hardness and Brittleness

Ionic solids are rigid but brittle. When a strong force shifts the layers of ions, similar charges may align and repel one another, which causes the crystal to break instead of bend.

Types

Ionic compounds can be divided into two main types based on the types of ions they contain: binary ionic compounds and polyatomic ionic compounds. ^[1-4]

1. Binary Ionic Compounds

These are made of two elements: a metal and a nonmetal. The metal becomes a cation, and the nonmetal becomes an anion.

Examples

Binary ionic compounds can be further classified into two types based on the charge of the metal cation: 

Type 1 compounds involve metals that have only one possible positive charge, such as sodium (Na⁺) or magnesium (Mg²⁺). These metals form predictable, fixed-charge ions, so the compound name does not need to indicate the charge. 

In contrast, Type 2 compounds contain metals that can form more than one type of positive ion, such as iron (Fe²⁺ or Fe³⁺) or copper (Cu⁺ or Cu²⁺). For these, the charge on the metal ion must be specified in the compound’s name using Roman numerals; for example, iron (III) oxide (FeO) or copper (II) nitrate (Cu₃N₂).

2. Polyatomic Ionic Compounds

These include at least one polyatomic ion – a group of atoms bonded covalently but acting as a single charged unit.

Examples

Difference Between Ionic and Covalent Compounds​

There is another type of chemical compound called a covalent compound, or molecular compound. These form when two or more nonmetal atoms share electrons instead of transferring them. This electron sharing creates a covalent bond and results in individual molecules, such as water (H₂O) or carbon dioxide (CO₂). ^[1-4]

Covalent compounds usually have lower melting and boiling points. They can exist as gases, liquids, or soft solids. Unlike ionic compounds, their solutions do not conduct electricity, since they do not form ions in water.