Molar MassMolar mass of elements and common gases
Many elements exist as molecules of two or more atoms, and a few simple compounds are gases at room temperature. For a gas, molar mass also gives density and tells whether it is lighter or heavier than air.
The seven elements that exist as two-atom molecules are hydrogen, nitrogen, oxygen, fluorine, chlorine, bromine and iodine. Use the molecule, not the single atom, whenever the element itself takes part in a reaction. Compare a molar mass with 29, the average for air, to see whether a gas rises or sinks.
This page lists 10 elements and common gases with their formulas and molar masses, lightest first. The lightest here is hydrogen at 2.02 g/mol and the heaviest is iodine at 253.80 g/mol. Tap a name for the full working, the percentage of each element and worked examples.
| Compound | Formula | Molar mass, g/mol |
|---|---|---|
| Hydrogen dihydrogen | H2 | 2.02 |
| Helium | He | 4.00 |
| Methane marsh gas | CH4 | 16.04 |
| Nitrogen dinitrogen | N2 | 28.01 |
| Oxygen dioxygen | O2 | 32.00 |
| Argon | Ar | 39.95 |
| Ozone trioxygen | O3 | 48.00 |
| Chlorine | Cl2 | 70.90 |
| Bromine | Br2 | 159.81 |
| Iodine | I2 | 253.80 |
Which values? This site uses the abridged standard atomic weights published by IUPAC (CIAAW), read on 12 November 2026. Your textbook or question paper may round them differently, for example H = 1 and O = 16, so its answer can differ slightly. In an exam, use the values the paper gives.
Not in the list?
Type any formula into the calculator. It handles brackets and water of crystallisation and shows the same working as these pages. The method by hand is on the step by step page.
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