About this dataset
A monatomic substance consists of individual atoms, while a diatomic molecule contains two bonded atoms. The familiar elemental diatomic molecules are hydrogen, nitrogen, oxygen, fluorine, chlorine, bromine, and iodine. They are often remembered as H₂, N₂, O₂, F₂, Cl₂, Br₂, and I₂.
The noble gases are the standard monatomic examples because their filled outer electron shells make isolated atoms comparatively stable. The table combines these classroom reference sets with identifiers and structure strings that can be reused in chemical software.
Why some elements are diatomic
For the seven commonly taught diatomic elements, pairing two atoms produces a more stable electronic arrangement than keeping the atoms separate under ordinary conditions. This does not mean that every two-atom molecule is an element: carbon monoxide and hydrogen chloride are also diatomic, but each contains two different elements.
State matters too. Bromine is a liquid and iodine is a solid near room temperature, yet their elemental forms are still Br₂ and I₂. “Diatomic” describes the number of atoms in the molecule, not its physical state.
How to use the list
The classification column separates diatomic molecules from the monatomic noble gases. Alkali halides such as sodium chloride and potassium bromide, and lithium hydride, are marked as diatomic in the gas phase only: their vapor contains discrete two-atom molecules, but the solids are ionic crystals rather than collections of molecules.
The type column then groups the diatomics into the seven elemental molecules, heteronuclear molecules such as CO and HCl, interhalogens such as ICl, and reactive or high-temperature species such as the hydroxyl radical and silicon monoxide. Formula identifies composition, while SMILES and PubChem CID provide machine-readable and database-ready references.
Compounds that are not diatomic
Some small molecules are often mistaken for diatomics. Hydrogen cyanide (HCN), nitrosyl chloride (NOCl), and dichlorine monoxide (Cl₂O) each have three atoms, and cyanogen (C₂N₂) has four. Magnesium oxide, calcium oxide, and boron nitride have two-element formulas but exist as extended ionic or covalent networks, so none of these appear in the table.
How this dataset was assembled
Every row is a two-atom species or a monatomic noble gas. Alkali halides and lithium hydride are labelled as gas-phase diatomics because they form discrete molecules only in the vapor and are ionic crystals as solids. Look-alikes with more than two atoms (HCN, C2N2, Cl2O, NOCl) and network or ionic solids such as MgO, CaO, and BN are excluded. Structures and InChIKeys are checked against the cited PubChem record.
Each row retains source identifiers where available. Molecular weight is calculated from the recorded formula, and the original names, structures, classifications, and database links remain visible so individual entries can be checked before reuse.
License and citation
This compilation is free to download and reuse under the Creative Commons Attribution 4.0 International license. Credit ProteinIQ and the original sources when you share it or publish results that use it.
Suggested citation: ProteinIQ (2026). List of diatomic molecules and monatomic elements, version 2026.10.05. https://proteiniq.io/datasets/diatomic-and-monatomic-elements
Sources and further reading
- PubChem: Canonical compound identifiers, names, formulas, structures, SMILES, and downloadable compound records. NCBI places no restrictions on PubChem data, though original submitters may retain rights in submitted records.
- NIST Chemistry WebBook: Reference physical chemistry data and diatomic constants for selected species. NIST SRD 69 should be cited and linked rather than mirrored wholesale.
- OpenStax Chemistry: Chemistry definitions used for educational polarity, bonding, organic chemistry, and molecule-class explanations.