Ionic compounds — Definition, Properties & Examples

Written by

Malcolm McKinsey

January 12, 2023

Fact-checked by

Paul Mazzola

What is an ionic compound?

Ionic compounds are pure substances consisting of chemically bonded ions. Examples include two-element compounds like table salt ($NaCl$) and polyatomic compounds like sodium sulfate ($N{A}_{2}S{O}_{4}$). All ionic compounds form crystal lattices.

Ionic compounds are pure substances formed primarily by positive and negative ion attraction. Some minor electron sharing always occurs, but the compound is chiefly formed by ions.

What are ionic compounds made of?

To fully understand ionic compounds, we need to understand what ionic compounds are composed of. Let’s review some basics of atomic structure:

• Nucleus – The center of every atom, consisting of positively charged protons and neutral neutrons

• Electron – The tiny, negatively charged particle orbiting the nucleus in the electron cloud and held to the atom by the positive-negative attraction between it and the nucleus’s proton

• Ion – An atom that has gained or lost an electron, creating a charged particle that is either positive (more protons than electrons) or negative (more electrons than protons)

The number of electrons lost or gained determines the ion's charge. Atoms lose and gain electrons as they react to become more stable.

Ionic vs. molecular compounds

Ionic compounds are pure substances formed from chemically bonded ions. Ionic bonds form ionic compounds. Covalent bonds form molecular compounds.

Most atoms join by covalent bonding, in which shared electrons form directional bonds. These are molecules.

Ionic compounds are electrically neutral and can form from just a few atoms (ions) or from large numbers of atoms (ions). Two-element ionic compounds and polyatomic ionic compounds are equally common.

Polyatomic ions are ions comprised of more than one atom. A good example is the ammonium ion made up of one nitrogen atom and four hydrogen atoms.

Ions that are negatively charged are called anions, pronounced “an-ions.” Common anions are non-metals. Ions with net positive charges are called cations, pronounced “cat-ions.” Metals are commonly cations.

Properties of ionic compounds

Ionic compounds form crystalline structures called crystal lattices. Common table salt ($NaCl$) forms this way, but so do ionic lattices such as:

• copper (II) sulfate ($CuS{O}_{4}{\left({H}_{2}O\right)}_{x}$) with the value of $x$ ranging from 0 through 5

• sodium borate ($N{a}_{2}{H}_{4}{B}_{4}{O}_{9}?n{H}_{2}O$) (known as borax)

• potassium permanganate ($KMn{O}_{4}$)

These are built up in repeated patterns of unit cells, as contrasted with molecules. One “piece” of an ionic compound that maintains all the entire compound's characteristics is called a unit.

In comparison, one “piece” of a covalently bonded compound that maintains all its entire compound characteristics is called a molecule.

Ionic compounds dissolve in polar solvents such as water. Water can dissolve ions and polar molecules. Non-polar solvents (hydrocarbons, chloroform) do not dissolve ionic compounds.

Other physical properties of ionic compounds:

• High melting points

• High boiling points

• They are generally hard and brittle

• They are electrically neutral

• They form efficient insulators

• High enthalpies of fusion (thermal energy needed to melt one mole of the solid form) and vaporization (thermal energy needed to vaporize one mole of the liquid form)

Ionic compound formula

While a chemical formula for a molecule gives a specific number of atoms in every molecule (${H}_{2}O$ tells us two hydrogen atoms and one oxygen atom make water, for instance), the chemical formula for an ionic compound cannot tell us how many ions are in each unit, only their ratio.

The symbols used to create a chemical formula are found on the periodic table.

Ionic compound examples

Two-element ionic compounds include sodium chloride or table salt. Here the sodium ion, $N{a}^{+}$, is a positive ion, so it is attracted to the chlorine atom (ion), $C{l}^{-}$, which has a negative charge, and the ionic bond is formed.

The opposite charges attract, and the oppositely charged ions form an ionic bond.

Table salt is an excellent ionic compound readily available for quick study. It is not dangerous, displays the typical crystal lattice structure of ionic compounds to the unaided eye, demonstrates solubility in a polar solvent (water) and insolubility in a non-polar solvent such as gasoline, and reveals the compound’s hardness and brittleness.

Ionic compounds list

Other two-element ionic compounds include:

• $C{a}_{3}{P}_{2}$ (calcium phosphide) – used as a rodenticide and in fireworks

• $Kl$ (potassium iodide) – an additive of table salt used to prevent goiter

• $L{i}_{3}N$ (lithium nitride) – a reducing agent

• $MgO$ (magnesium oxide) – used as an antacid and laxative

• $NaF$ (sodium fluoride) – an ingredient used in toothpaste

• $BaO$ (barium oxide) – used in cathode ray tubes, crown glass, and catalysts

Many polyatomic ionic compounds exist. Here are a few:

• $Al{\left(OH\right)}_{3}$ (aluminum hydroxide) – used in antacids

• $CaC{O}_{3}$ (calcium carbonate) – used in antacids

• ${K}_{3}P{O}_{4}$ (potassium phosphate) – a food additive

• $Mg{\left(OH\right)}_{2}$ (magnesium hydroxide) – used in antacids

• $MgS{O}_{4}$ (magnesium sulfate) – an additive of purified water

• $N{a}_{2}C{O}_{3}$ (sodium carbonate) – washing soda

• $NaHC{O}_{3}$ (sodium bicarbonate) – baking soda

• $N{a}_{2}HP{O}_{4}$ (sodium hydrogen phosphate) – it prevents caking and is added to many powdered products

• $NaOCl$ (sodium hypochlorite) – household bleach

• $NaOH$ (sodium hydroxide) – drain cleaner

• $N{a}_{2}S{O}_{3}$ (sodium sulfite) – a preservative

Ionic compound quiz

Ionic compounds can confuse anyone. See if you have a grasp of the ideas by answering these questions. Then check your work against our answers below.

1. Is salt an ionic compound?

2. Name three ionic compounds.

3. Name a common two-element ionic compound.

4. Covalent compounds form molecules. What do ionic compounds form?

5. What is the typical structure of an ionic compound?

6. Please list three characteristics of ionic compounds.

7. Please name some common uses of ionic compounds.

If you struggled with any of your answers, spend some time reviewing this article and do additional research on your own to firm up your understanding.

1. Yes, salt is an ionic compound.

2. Three ionic compounds are $NaCl$ (table salt), $NaOCl$ (bleach), and $NaHC{O}_{3}$ (baking soda).

3. A common two-element ionic compound is sodium chloride, or table salt, $NaCl$.

4. Covalent compounds form molecules, and ionic compounds form units.

5. The typical structure of an ionic compound is a crystal lattice.

6. A polyatomic compound is sodium bicarbonate, $NaHC{O}_{3}$ (baking soda).

7. Three characteristics of ionic compounds you might have listed are that they are electrically neutral, dissolve only in polar solvents, and, as solids, are hard and brittle.