Diatomic molecules have only two atoms. If a diatomic molecule is homonuclear, both of its atoms have the same nuclear composition. Each atom has the same number of protons in its nucleus and the same number of neutrons. As a result, both are atoms of the same isotope of the same element. Not many humonuclear diatomic molecules exist, so it is easy to remember them.
The same element may form two or more different homonuclear diatomic molecules. For example, O2 may consist of two oxygen atoms with an atomic weight of 16, or both oxygen atoms may have an atomic weight of 18. It simplifies matters if isotopes are not taken into consideration. It is far easier to commit to memory only the basic elements that form homonuclear diatomic molecules. Even deuterium may be ignored, since it is an isotope of hydrogen.
The mind more easily grasps and remembers the homonuclear diatomic molecules if they are arranged in a logical order. The following exhaustive list orders them according to the atomic number of their elements: hydrogen, nitrogen, oxygen, fluorine, chlorine, bromine and iodine. An alternate arrangement may use the alphabetical order of these same elements. Choose a logical order convenient for you and stick to it.
Classes of Elements
The homonuclear diatomic molecules fall into three rough classes that are easily remembered. Hydrogen is in a class by itself. It is the simplest element with only one proton. A second class consists of nitrogen and oxygen, the two chief gases in the atmosphere. The third class consists of the most abundant halogens: fluorine, chlorine, bromine and iodine. At present, it is not necessary to remember a fifth halogen called astatine. Because of the rarity and the short half-life of this radioactive element, no one has succeeded in synthesizing diatomic astatine.
By definition, a mnemonic is a memory aid. The chemical symbols of the elements that form homonuclear diatomic molecules may serve as the basis of a mnemonic device that helps the mind remember the names of the molecules. Suppose that the chemical symbols are arranged as follows: H, N, O, F Cl, Br and I. Using each of these symbols as the first letter or letters of a word, the following mnemonic may be formed: “Healthy nerves originate from clear brown iodine."