Section 4: Matter and Energy
Atoms, Elements, Molecules, and Compounds

Molecules and compounds

Molecules have been described as “the smallest particle[s] in a chemical element or compound that [have] the chemical properties of that element or compound” (Rouse, 2008, par. 1). They have also been described as forming “when two or more atoms join together chemically,” (Gagnon, n.d., par. 1).  Also, “[m]olecule is the general term used to describe atoms connected by chemical bonds” (Rader, 2014, par. 1). All compounds are molecules, but not all molecules are compounds (if they happen to have atoms of the same element).  

Diatomic molecules are molecules composed of only two atoms of either the same or different elements. Earth’s trophosphere is composed of approximately 99 percent diatomic molecules with 21 percent oxygen and 78 percent nitrogen. Most diatomic molecules are not made up of the same kind of elements; for example, the diatomic molecules that comprise the chemical compounds nitric acid, carbon monoxide, and hydrogen chloride are made up of two different elements. However, not every diatomic molecule comprises a chemical compound. Look at the illustration below.

Diatomic Molecules

These are the seven diatomic elements on the periodic table. Read more about them below.