By classifying compounds, we can make general predictions about their properties and reactivity. Compounds can be classified as ionic or covalent:

1. Ionic compounds consist of cations and anions whose total charges cancel each other out. Because most metals form cations and most nonmetals form anions, formulas of ionic compounds typically include both metal(s) and nonmetal(s).
2. Covalent bonds form when two or more nonmetals share electrons in covalent bonds. Thus, the formula of a covalent compound will include only nonmetals.

Differentiating Between Ionic and Covalent Compounds

Compounds between metal and nonmetal elements are usually ionic. Thus, the periodic table can help us recognize ionic compounds. For example, CaBr₂ contains a metallic element (calcium, a group 2 metal) and a nonmetallic element (bromine, a group 17 nonmetal). Therefore, we can assume it is an ionic compound.

In some cases, you may see a metal combined with multiple nonmetals. The group of nonmetals is called a polyatomic ion. Polyatomic ions consist of a group of nonmetals which are bonded together and share an overall charge. A polyatomic ion is charged, so it is an ion, and will be found in ionic compounds. For example, baking soda, a common ingredient in baked goods, has the formula NaHCO₃. The cation is the sodium ion, Na⁺. The anion is the polyatomic ion bicarbonate, HCO₃^–. (Remember that the convention for writing formulas for ionic compounds is not to include the ionic charge when the cation and anion are combined in a compound.) Baking soda is an ionic compound since it consists of these ions.

Whereas ionic compounds are usually formed when a metal and a nonmetal combine, covalent compounds are usually formed by a combination of nonmetals. We can also use the periodic table to help us recognize covalent compounds. For example, the compound NO₂ contains two elements that are both nonmetals (nitrogen, from group 15, and oxygen, from group 16). It is not an ionic compound; it belongs to the category of covalent compounds. You have already seen examples of substances that contain covalent bonds. One common example water (H₂O). You can tell from its formula that it is not an ionic compound; it is not composed of a metal and a nonmetal.

While we can use the positions of a compound’s elements in the periodic table to predict whether it is ionic or covalent at this point in our study of chemistry, you should be aware that this is a very simplistic approach that does not account for a number of interesting exceptions. Shades of gray exist between ionic and covalent compounds, and some compounds found in food have structures and properties that are a sort of hybrid between ionic and molecules. You’ll learn more about those later.

Example: Identify Compounds as Either Ionic or Covalent

Identify the following compounds as wither ionic or covalent:

1. KI, the compound used as a source of iodine in table salt
2. H₂O₂, the bleach and disinfectant hydrogen peroxide
3. CHCl₃, the anesthetic chloroform
4. Li₂CO₃, a source of lithium in antidepressants

Solution

1. Potassium (group 1) is a metal, and iodine (group 17) is a nonmetal; KI is predicted to be ionic.
2. Hydrogen (group 1) is a nonmetal, and oxygen (group 16) is a nonmetal; H₂O₂ is predicted to be covalent.
3. Carbon (group 14) is a nonmetal, hydrogen (group 1) is a nonmetal, and chlorine (group 17) is a nonmetal; CHCl₃ is predicted to be covalent.
4. Lithium (group 1) is a metal, and carbonate is a polyatomic ion; Li₂CO₃ is predicted to be ionic.

Attributions

This page is based on “Chemistry 2e” by Paul Flowers, Klaus Theopold, Richard Langley, William R. Robinson, PhD, Openstax which is licensed under CC BY 4.0. Access for free at https://openstax.org/books/chemistry-2e/pages/1-introduction

This page is based on “The Basics of General, Organic, and Biological Chemistry” by David W Ball, John W Hill, Rhonda J Scott, Saylor which is licensed under CC BY-NC-SA 4.0. Access for free at http://saylordotorg.github.io/text_the-basics-of-general-organic-and-biological-chemistry/index.html