A compound is any combination of two or more different types of atoms (a molecule is a combination of any two atoms; they do not need to be different). There are several different types of compounds, and the characteristics of compounds come from the type of bonds that they form; ionic compounds are formed from ionic bonds.
Ionic Compound Definition
Ionic compounds are compounds in which the atoms are held together by ionic bonds. An ionic bond occurs when two oppositely charged ions are attracted. An ion is an atom that has either gained or lost an electron, and thus has a positive or negative charge; ions have different chemical properties than the neutral (as listed on the periodic table) form of the atom. Ionic compounds are made up of at least one metallic element and one nonmetallic element.
Ionic compounds are solids at room temperature. Solidness is a sate of matter in which the material is relatively resistant to change. Additionally, ionic compounds are generally soluble in water, though being soluble in water does not change the solid state of a compound. An example of ionic compounds that are solids is common table salt, which forms with a sodium ion and a chlorine ion. Note that solids that contain carbon are not ionic bonds; carbon forms a covalent bond.
Because of the presence of a metallic element, most ionic compounds retain the physical characteristics of metals, chief of which is that they are good conductors of heat and electricity. However, the solid form of an ionic compound is not nearly as good at conducting electricity as when it is dissolved in water. Additionally, metals have higher density than nonmetallic substances, and they often contain luster (which is when light reflects off of a substance).
Ionic bonds are relatively stable, which is part of the reason why ionic compounds are generally solid. As a result, ionic compounds have higher boiling and melting points because their bonds are resistant to change (boiling points and melting points are the temperatures at which a solid changes its state to a gas or liquid, respectively). The energy that keeps the positive and negative ions together in such a strong bond is known as "lattice energy."