Ionic Compounds

When electrons are transferred and ions form, ionic bonds result. Because opposite charges attract (while like charges repel), cations and anions attract each other, forming ionic bonds. The resulting compounds are called ionic compounds and are the primary subject of this section.

We have already encountered some chemical formulas for simple ionic compounds such as table satl. The chemical formula of table salt, also known as sodium chloride, is NaCl. A chemical formula is a concise list of the elements in a compound and the ratios of these elements. Note: By convention, assume that there is only one atom if a subscript is not present. We do not use 1 as a subscript.

Ionic compounds exist as alternating positive and negative ions in regular, three-dimensional arrays called crystals. As you can see in the figure below, there are no individual NaCl “particles” in the array; instead, there is a continuous lattice of alternating sodium and chloride ions. However, we can use the ratio of sodium ions to chloride ions, expressed in the lowest possible whole numbers, as a way of describing the compound. In the case of sodium chloride, the ratio of sodium ions to chloride ions, expressed in lowest whole numbers, is 1:1, so we use NaCl (one Na symbol and one Cl symbol) to represent the compound. Thus, NaCl is the chemical formula for sodium chloride, which is a concise way of describing the relative number of different ions in the compound.

The figure below shows the crystal structure of NaCl. Note that no single ion is exclusively associated with any other single ion. Each ion is surrounded by multiple ions of opposite charge.

Two diagrams are shown and labeled “a” and “b.” Diagram a shows a cube made up of twenty-seven alternating purple and green spheres. The purple spheres are smaller than the green spheres. Diagram b shows the same spheres, but this time, they are spread out and connected in three dimensions by white rods. The purple spheres are labeled “N superscript postive sign” while the green are labeled “C l superscript negative sign.”
The atoms in sodium chloride (common table salt) are arranged to (a) maximize opposite charges interacting. The smaller spheres represent sodium ions, the larger ones represent chloride ions. In the expanded view (b), the geometry can be seen more clearly. Note that each ion is “bonded” to all of the surrounding ions—six in this case.

The formula for an ionic compound follows several conventions. First, the cation is written before the anion. Because most metals form cations and most nonmetals form anions, formulas typically list the metal first and then the nonmetal. Second, charges are not written in a formula. Remember that in an ionic compound, the component species are ions, not neutral atoms, even though the formula does not contain charges. Finally, the proper formula for an ionic compound always obeys the following rule: the total positive charge must equal the total negative charge. Different numbers of cations or anions may be needed to balance the total positive and negative charges in the compound. This rule is ultimately based on the fact that matter is, overall, electrically neutral.

Physical Properties of Ionic Compounds

Melting Points

Because of the many simultaneous attractions between cations and anions that occur, ionic crystal lattices are very strong. The process of melting an ionic compound requires the addition of large amounts of energy in order to break all of the ionic bonds in the crystal. For example, sodium chloride has a melting temperature of about 800 °C, which is much, much higher than any temperature that can be achieved with household cooking equipment, like an oven or stovetop. As a comparison, the covalent compound water melts at 0 °C, below room temperature.


Ionic compounds are generally hard, but brittle. Why? It takes a large amount of mechanical force, such as striking a crystal with a hammer, to force one layer of ions to shift relative to its neighbor. However, when that happens, it brings ions of the same charge next to each other (see below). The repulsive forces between like-charged ions cause the crystal to shatter. When an ionic crystal breaks, it tends to do so along smooth planes because of the regular arrangement of the ions.

(A) A symbolic 2-D cross section of a sodium chloride. (B) A hammer hits the 2-D cross section, resulting in like-charge ions being moved close together, which leads to charge repulsion.
(A) The sodium chloride crystal is shown in two dimensions. (B) When struck by a hammer, the negatively-charged chloride ions are forced near each other and the repulsive force causes the crystal to shatter.


Another characteristic property of ionic compounds is their electrical conductivity. The figure below shows three experiments in which two electrodes that are connected to a light bulb are placed in beakers containing three different substances.

(A) Two electrodes are connected to a light bulb and are placed in a beaker containing distilled water. (B) Two electrodes are connected to a light bulb and are placed in a beaker containing an ionic solid. (C) Two electrodes are connected to a light bulb and are placed in a beaker containing water with ions dissolved in it.
(A) Distilled water does not conduct electricity. (B) A solid ionic compound also does not conduct. (C) A water solution of an ionic compound conducts electricity well.

In the first beaker, distilled water does not conduct a current because water is a covalent compound that contains no charged ions. In the second beaker, solid sodium chloride also does not conduct a current. Despite being ionic and thus composed of charged ion particles, the solid crystal lattice does not allow the ions to move between the electrodes. Mobile charged particles are required for the circuit to be complete and the light bulb to light up. In the third beaker, the ionic compound NaCl has been dissolved into the water. Now the crystal lattice has been broken apart and the individual positive and negative ions can move. Cations move to one electrode, while anions move to the other, allowing electricity to flow (see figure below). All ionic compounds conduct an electric current when dissolved in water. In fact, the electrolytes that you may have noticed in an ingredient list of an energy drink are ionic compounds dissolved in water! They are called electrolytes since the charged ions conduct electricity when dissolved in water. Electrolytes, or dissolved ionic compounds, are very important for the human body to function properly.

Two electrodes are connected to a battery and are placed in a beaker containing water with ions dissolved in it. Due to the electrical current, the positive cations travel towards the negative electrode, and the negative anions travel towards the positive electrode.
In an ionic solution, the A+ ions migrate toward the negative electrode, while the B− ions migrate toward the positive electrode.


This page is based on “Chemistry 2e” by Paul Flowers, Klaus Theopold, Richard Langley, William R. Robinson, PhDOpenstax which is licensed under CC BY 4.0. Access for free at

This page is based on “The Basics of General, Organic, and Biological Chemistry” by David W Ball, John W Hill, Rhonda J ScottSaylor which is licensed under CC BY-NC-SA 4.0. Access for free at


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