A diatomic element is a molecule of an element consisting of two atoms. It is a form of homonuclear diatomic molecule. There are only 7 diatomic elements in total and only 5 diatomic elements at standard temperature and pressure (STP). The diatomic elements are hydrogen, nitrogen, oxygen, fluorine, chlorine, bromine, and iodine.
List of the 7 Diatomic Elements
The following 5 element gases are found as diatomic molecules at room temperature and pressure:
The homonuclear diatomic gases are also known as “molecular gases” or “elemental gases.”
Bromine and iodine commonly exist in liquid form, but also as diatomic gases at slightly higher temperatures, making a total of 7 diatomic elements.
- Bromine – Br2
- Iodine – I2
All of the diatomic elements are types of nonmetals. Fluorine, chlorine, bromine, and iodine are all halogens. It’s possible astatine (atomic number 85, element symbol At) and tennessine (atomic number 117, element symbol Ts) may form diatomic molecules, but insufficient quantities have been produced to verify this behavior. Some of these elements form other stable molecules. For example, oxygen also forms ozone (O3), which is triatomic.
Other elements can form diatomic molecules besides the seven on this list. However, these molecules are not very stable, so their chemical bonds are readily broken. Diphosphorus (P2), disulfur (S2), dilithium (Li2), and disodium (Na2) all occur when their solids are evaporated, but these diatomic elements only exist as gases that repolymerize when cooled. Ditungsten (W2) and dimolybdenum (Mo2) also occur. These elements are interesting because they form sextuple bonds with each other in the gas phase! Under special conditions, rubidium can form dirubidium (Rb2) and trirubidium (Rb3).
How to Remember the Diatomic Elements
An easy mnemonic device is:
Have No Fear Of Ice Cold Beer
The diatomic elements are the –ine halogens (fluorine, chlorine, bromine, iodine) and elements with a –gen ending (hydrogen, oxygen, nitrogen). Astatine is another halogen, but its behavior is not known.
Other mnemonics are BrINClHOF (pronounced “Brinklehof”), HOFBrINCl (pronounced “Hofbrinkle”), and HONClBrIF (pronounced “Honkelbrif”). These mnemonic devices aid in recalling which elements are diatomic, but not their order on the periodic table.
Other Diatomic Molecules
While diatomic elements are uncommon, diatomic molecules readily occur. Common examples include sodium chloride (NaCl), carbon monoxide (CO), and nitric oxide (NO). Around 99% of the Earth’s atmosphere consists of diatomic molecules (nitrogen and oxygen).
- Huber, K. P.; Herzberg, G. (1979). Molecular Spectra and Molecular Structure IV. Constants of Diatomic Molecules. New York: Van Nostrand: Reinhold.
- Sherman, Alan (1992). Chemistry and Our Changing World. Prentice Hall. ISBN 9780131315419.
- Lu, Z.W.; Wang, Q.; He, W.M.; Ma, Z.G. (July 1996). “New parametric emissions in diatomic sodium molecules”. Applied Physics B. 63 (1): 43–46. doi:10.1007/BF01112836