A diatomic molecule is a chemical unit composed of exactly two atoms that are chemically bonded together. The term “diatomic” comes from the Greek roots “di” (two) and “atomic” (referring to atoms). These two-atom structures are common and represent a significant portion of the substances that make up the atmosphere and the broader natural world.
The Seven Naturally Occurring Diatomic Elements
The most commonly referenced diatomic molecules are the seven elements that naturally exist as two-atom units under standard conditions. These seven elements are hydrogen (H2), nitrogen (N2), oxygen (O2), fluorine (F2), chlorine (Cl2), bromine (Br2), and iodine (I2). They are often referred to as the elemental gases, though not all are gases at room temperature. A simple mnemonic phrase, like “Have No Fear Of Ice Cold Beer,” can help remember the list.
These seven elements are almost always found in the bonded X2 form when uncombined with other elements. Five of them—hydrogen, nitrogen, oxygen, fluorine, and chlorine—exist as gases at standard room temperature and pressure. Bromine is a reddish-brown liquid, while iodine is typically a solid that readily sublimes. Nitrogen and oxygen are particularly important, constituting approximately 99 percent of Earth’s atmosphere in their diatomic forms.
Understanding the Chemical Drive for Stability
The fundamental reason atoms bond together to form diatomic molecules is the chemical drive to achieve a lower, more stable energy state. Atoms seek to fill their outermost electron shells, mimicking the stable configuration of noble gases. For most elements, this involves having eight electrons in the outer shell, known as the octet rule. Hydrogen is an exception, requiring only two electrons to achieve stability (the duet rule).
When two atoms, such as chlorine, have an unfilled outer shell, they share valence electrons to satisfy their requirement for stability. This sharing forms a covalent bond, which effectively lowers the overall energy of the system. The resulting diatomic molecule is significantly more stable and less reactive than the two individual, unbonded atoms. For instance, nitrogen atoms form a triple bond in N2 by sharing three pairs of electrons to satisfy the octet rule.
Distinguishing Homonuclear and Heteronuclear Diatomics
Diatomic molecules are categorized based on whether the two bonded atoms are the same or different elements. Homonuclear diatomic molecules are composed of two atoms of the same element, such as O2 or H2. The seven naturally occurring diatomic elements are all examples of homonuclear diatomics. Since the nuclei are identical, the electron density is distributed evenly, resulting in a nonpolar covalent bond.
In contrast, heteronuclear diatomic molecules are formed when two atoms of different elements bond together. This category includes common substances like carbon monoxide (CO), hydrogen chloride (HCl), and nitric oxide (NO). Because the two atoms have different electronegativities, the shared electrons are pulled more closely toward one atom. This unequal sharing creates a polar covalent bond, resulting in one side of the molecule having a slightly negative charge and the other a slightly positive charge.