Ionic compound are made up of ions rather than molecules. Instead of sharing electrons in covalent bonds, ionic compound atoms transfer electrons from one atom to another to form an ionic bond that relies on electrostatic attraction to keep the atoms together. Covalently bonded molecules share electrons and act as a stable, single entity while an ionic bond results in independent ions that have a positive or negative charge. Because of their special structure, ionic compounds have unique properties and react easily with other ionic compounds when placed in solution.

The atoms of elements such as hydrogen, sodium and potassium have only one electron in their outermost electron shell while atoms such as calcium, iron and chromium have several loosely held electrons. These atoms can donate the electrons in their outermost shell to atoms that need electrons to complete their electron shells.

The atoms of chlorine and bromine have seven electrons in their outermost shell where there is room for eight. Oxygen and sulfur atoms each need two electrons to complete their outermost shells. When the outermost shell of an atom is complete, the atom becomes a stable ion.

In chemistry, ionic compounds are formed when donor atoms transfer electrons to receiving atoms. For example, a sodium atom with one electron in its third shell can react with a chlorine atom that needs an electron to form NaCl. The electron from the sodium atom transfers to the chlorine atom. The outermost shell of the sodium atom, which is now the second shell, is full with eight electrons, while the outermost shell of the chlorine atom is also full with eight electrons. The oppositely charged sodium and the chlorine ions attract each other to form the NaCl ionic bond.

In another example, two potassium atoms, each with one electron in their outermost shells, can react with a sulfur atom that needs two electrons. The two potassium atoms transfer their two electrons to the sulfur atom to form the ionic compound potassium sulfide.

Molecules can themselves form ions and react with other ions to create ionic bonds. Such compounds behave as ionic compounds as far as the ionic bonds are concerned, but they also have covalent bonds. For example, nitrogen can form covalent bonds with four hydrogen atoms to produce the ammonium ion but the NH₄ molecule has one extra electron. As a result, NH₄ reacts with sulfur to form (NH₄)₂S. The bond between NH₄ and the sulfur atom is ionic while the bonds between the nitrogen atom and the hydrogen atoms are covalent.

Ionic compounds have special characteristics because they are made up of individual ions rather than molecules. When dissolved in water, the ions break apart or dissociate from each other. They can then easily take part in chemical reactions with other ions that are dissolved as well.

Because they carry an electric charge, they conduct electricity when dissolved, and ionic bonds are strong, needing a lot of energy to break them. Ionic compounds have high melting and boiling points, may form crystals and are generally hard and brittle. With these characteristics distinguishing them from many other compounds based on covalent bonds, identifying ionic compounds can help anticipate how they will react and what their properties will be.