Ionic compounds are composed of ions, which are atoms that have lost or gained electrons. When an atom loses or gains electrons, it becomes charged and gains a net positive or negative charge. The positive ions (cations) and negative ions (anions) are attracted to each other and form ionic compounds.

The elements in ionic compounds are typically metals and nonmetals. Metals tend to lose electrons and form positive ions, while nonmetals tend to gain electrons and form negative ions. Some of the most common elements found in ionic compounds include:

* Metals: Sodium (Na), potassium (K), calcium (Ca), magnesium (Mg), and aluminum (Al)

* Nonmetals: Fluorine (F), chlorine (Cl), bromine (Br), iodine (I), oxygen (O), nitrogen (N), and sulfur (S)

These elements can combine in a variety of ways to form different ionic compounds. For example, sodium and chloride ions combine to form sodium chloride (NaCl), while calcium and fluoride ions combine to form calcium fluoride (CaF$_2$).

Ionic compounds are important in many everyday applications. They are used in a variety of products, including table salt, baking soda, toothpaste, and fertilizer.