# List of diatomic molecules and monatomic elements

A reference list of true diatomic molecules (the seven elemental diatomics, heteronuclear and interhalogen molecules, and reactive gas-phase species) and the monatomic noble gases, with formulas and PubChem identifiers.

Rows43

Last updatedOctober 5, 2026

Version2026.10.05

License[CC BY 4.0](https://creativecommons.org/licenses/by/4.0/)

FormatsCSV, JSON

SourcesPubChem, NIST Chemistry WebBook, OpenStax Chemistry

Citation

[CSV](https://proteiniq.io/datasets/diatomic-and-monatomic-elements/download?format=csv)[JSON](https://proteiniq.io/datasets/diatomic-and-monatomic-elements/download?format=json)

|  |  |  |  |  |  |  |  |
| --- | --- | --- | --- | --- | --- | --- | --- |
| Hydrogen | H2 | 2.02 | Diatomic molecule | Elemental diatomic (homonuclear) | `[HH]` | [783](https://pubchem.ncbi.nlm.nih.gov/compound/783) |  |
| Nitrogen | N2 | 28.01 | Diatomic molecule | Elemental diatomic (homonuclear) | `N#N` | [947](https://pubchem.ncbi.nlm.nih.gov/compound/947) |  |
| Oxygen | O2 | 32.00 | Diatomic molecule | Elemental diatomic (homonuclear) | `O=O` | [977](https://pubchem.ncbi.nlm.nih.gov/compound/977) |  |
| Fluorine | F2 | 38.00 | Diatomic molecule | Elemental diatomic (homonuclear) | `FF` | [24524](https://pubchem.ncbi.nlm.nih.gov/compound/24524) |  |
| Chlorine | Cl2 | 70.90 | Diatomic molecule | Elemental diatomic (homonuclear) | `ClCl` | [24526](https://pubchem.ncbi.nlm.nih.gov/compound/24526) |  |
| Bromine | Br2 | 159.81 | Diatomic molecule | Elemental diatomic (homonuclear) | `BrBr` | [24408](https://pubchem.ncbi.nlm.nih.gov/compound/24408) |  |
| Iodine | I2 | 253.80 | Diatomic molecule | Elemental diatomic (homonuclear) | `II` | [807](https://pubchem.ncbi.nlm.nih.gov/compound/807) |  |
| Helium | He | 4.00 | Monatomic element | Noble gas | `[He]` | [23987](https://pubchem.ncbi.nlm.nih.gov/compound/23987) |  |
| Neon | Ne | 20.18 | Monatomic element | Noble gas | `[Ne]` | [23935](https://pubchem.ncbi.nlm.nih.gov/compound/23935) |  |
| Argon | Ar | 39.95 | Monatomic element | Noble gas | `[Ar]` | [23968](https://pubchem.ncbi.nlm.nih.gov/compound/23968) |  |
| Krypton | Kr | 83.80 | Monatomic element | Noble gas | `[Kr]` | [5416](https://pubchem.ncbi.nlm.nih.gov/compound/5416) |  |
| Xenon | Xe | 131.29 | Monatomic element | Noble gas | `[Xe]` | [23991](https://pubchem.ncbi.nlm.nih.gov/compound/23991) |  |
| Radon | Rn | 222.00 | Monatomic element | Noble gas | `[Rn]` | [24857](https://pubchem.ncbi.nlm.nih.gov/compound/24857) |  |
| Carbon monoxide | CO | 28.01 | Diatomic molecule | Heteronuclear diatomic molecule | `[C-]#[O+]` | [281](https://pubchem.ncbi.nlm.nih.gov/compound/281) |  |
| Nitric oxide | NO | 30.01 | Diatomic molecule | Heteronuclear diatomic molecule | `[N]=O` | [145068](https://pubchem.ncbi.nlm.nih.gov/compound/145068) |  |
| Hydrogen fluoride | HF | 20.01 | Diatomic molecule | Heteronuclear diatomic molecule | `F` | [14917](https://pubchem.ncbi.nlm.nih.gov/compound/14917) |  |
| Hydrogen chloride | HCl | 36.46 | Diatomic molecule | Heteronuclear diatomic molecule | `Cl` | [313](https://pubchem.ncbi.nlm.nih.gov/compound/313) |  |
| Hydrogen bromide | HBr | 80.91 | Diatomic molecule | Heteronuclear diatomic molecule | `Br` | [260](https://pubchem.ncbi.nlm.nih.gov/compound/260) |  |
| Hydrogen iodide | HI | 127.91 | Diatomic molecule | Heteronuclear diatomic molecule | `I` | [24841](https://pubchem.ncbi.nlm.nih.gov/compound/24841) |  |
| Chlorine monofluoride | ClF | 54.45 | Diatomic molecule | Interhalogen diatomic molecule | `FCl` | [123266](https://pubchem.ncbi.nlm.nih.gov/compound/123266) |  |
| Bromine monochloride | BrCl | 115.35 | Diatomic molecule | Interhalogen diatomic molecule | `ClBr` | [61697](https://pubchem.ncbi.nlm.nih.gov/compound/61697) |  |
| Iodine monochloride | ICl | 162.35 | Diatomic molecule | Interhalogen diatomic molecule | `ClI` | [24640](https://pubchem.ncbi.nlm.nih.gov/compound/24640) |  |
| Iodine monobromide | IBr | 206.80 | Diatomic molecule | Interhalogen diatomic molecule | `BrI` | [82238](https://pubchem.ncbi.nlm.nih.gov/compound/82238) |  |
| Bromine monofluoride | BrF | 98.90 | Diatomic molecule | Interhalogen diatomic molecule | `FBr` | [139632](https://pubchem.ncbi.nlm.nih.gov/compound/139632) |  |
| Iodine monofluoride | IF | 145.90 | Diatomic molecule | Interhalogen diatomic molecule | `FI` | [139637](https://pubchem.ncbi.nlm.nih.gov/compound/139637) |  |
| Hydroxyl radical | OH | 17.01 | Diatomic molecule | Reactive or high-temperature diatomic | `[OH]` | [157350](https://pubchem.ncbi.nlm.nih.gov/compound/157350) |  |
| Cyano radical | CN | 26.02 | Diatomic molecule | Reactive or high-temperature diatomic | `[C]#N` | [5359238](https://pubchem.ncbi.nlm.nih.gov/compound/5359238) |  |
| Chlorine monoxide | ClO | 51.45 | Diatomic molecule | Reactive or high-temperature diatomic | `[O]Cl` | [166686](https://pubchem.ncbi.nlm.nih.gov/compound/166686) |  |
| Sulfur monoxide | SO | 48.06 | Diatomic molecule | Reactive or high-temperature diatomic | `O=S` | [114845](https://pubchem.ncbi.nlm.nih.gov/compound/114845) |  |
| Silicon monoxide | SiO | 44.08 | Diatomic molecule | Reactive or high-temperature diatomic | `O=[Si]` | [66241](https://pubchem.ncbi.nlm.nih.gov/compound/66241) |  |
| Phosphorus monoxide | PO | 46.97 | Diatomic molecule | Reactive or high-temperature diatomic | `O=[P]` | [6857426](https://pubchem.ncbi.nlm.nih.gov/compound/6857426) |  |
| Carbon monosulfide | CS | 44.07 | Diatomic molecule | Reactive or high-temperature diatomic | `[C-]#[S+]` | [108054](https://pubchem.ncbi.nlm.nih.gov/compound/108054) |  |
| Boron monoxide | BO | 26.81 | Diatomic molecule | Reactive or high-temperature diatomic | `[B]=O` | [6327189](https://pubchem.ncbi.nlm.nih.gov/compound/6327189) |  |
| Aluminum monoxide | AlO | 42.98 | Diatomic molecule | Reactive or high-temperature diatomic | `O=[Al]` | [142249](https://pubchem.ncbi.nlm.nih.gov/compound/142249) |  |
| Lithium hydride | LiH | 7.95 | Diatomic molecule (gas phase only) | Ionic compound, a crystal lattice at room temperature | `[H-].[Li+]` | [62714](https://pubchem.ncbi.nlm.nih.gov/compound/62714) |  |
| Lithium fluoride | LiF | 25.94 | Diatomic molecule (gas phase only) | Ionic compound, a crystal lattice at room temperature | `[Li+].[F-]` | [224478](https://pubchem.ncbi.nlm.nih.gov/compound/224478) |  |
| Lithium chloride | LiCl | 42.39 | Diatomic molecule (gas phase only) | Ionic compound, a crystal lattice at room temperature | `[Li+].[Cl-]` | [433294](https://pubchem.ncbi.nlm.nih.gov/compound/433294) |  |
| Sodium fluoride | NaF | 41.99 | Diatomic molecule (gas phase only) | Ionic compound, a crystal lattice at room temperature | `[F-].[Na+]` | [5235](https://pubchem.ncbi.nlm.nih.gov/compound/5235) |  |
| Sodium chloride | NaCl | 58.44 | Diatomic molecule (gas phase only) | Ionic compound, a crystal lattice at room temperature | `[Na+].[Cl-]` | [5234](https://pubchem.ncbi.nlm.nih.gov/compound/5234) |  |
| Sodium iodide | NaI | 149.89 | Diatomic molecule (gas phase only) | Ionic compound, a crystal lattice at room temperature | `[Na+].[I-]` | [5238](https://pubchem.ncbi.nlm.nih.gov/compound/5238) |  |
| Potassium fluoride | KF | 58.10 | Diatomic molecule (gas phase only) | Ionic compound, a crystal lattice at room temperature | `[F-].[K+]` | [522689](https://pubchem.ncbi.nlm.nih.gov/compound/522689) |  |
| Potassium chloride | KCl | 74.55 | Diatomic molecule (gas phase only) | Ionic compound, a crystal lattice at room temperature | `[Cl-].[K+]` | [4873](https://pubchem.ncbi.nlm.nih.gov/compound/4873) |  |
| Potassium bromide | KBr | 119.00 | Diatomic molecule (gas phase only) | Ionic compound, a crystal lattice at room temperature | `[K+].[Br-]` | [253877](https://pubchem.ncbi.nlm.nih.gov/compound/253877) |  |

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## 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](https://creativecommons.org/licenses/by/4.0/) 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](https://pubchem.ncbi.nlm.nih.gov/): 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](https://webbook.nist.gov/chemistry/): Reference physical chemistry data and diatomic constants for selected species. NIST SRD 69 should be cited and linked rather than mirrored wholesale.
- [OpenStax Chemistry](https://openstax.org/details/books/chemistry-2e): Chemistry definitions used for educational polarity, bonding, organic chemistry, and molecule-class explanations.

## Frequently asked questions