Fluorine gas has a density of 1.696 g/L at standard temperature and pressure (0 °C and 1 atmosphere). That makes it about 1.3 times heavier than air, so it tends to sink and collect near the ground. Depending on whether fluorine is in its gaseous, liquid, or solid state, its density changes dramatically.
Density as a Gas
At room conditions, fluorine exists as a pale yellow gas made up of diatomic molecules (F₂). Each molecule has a molecular weight of roughly 38 grams per mole, based on fluorine’s atomic weight of 18.998. That molecular weight is what gives fluorine gas its density of 1.696 g/L at STP. For comparison, ordinary air sits around 1.29 g/L, and oxygen is about 1.43 g/L. Fluorine is heavier than both.
Because fluorine gas is denser than air, it doesn’t rise and disperse the way lighter gases like hydrogen or helium do. Instead, it settles into low-lying areas. This is one reason industrial facilities that handle fluorine use specialized ventilation, gas detection sensors (set to trigger alarms at concentrations as low as 0.1 ppm), and enclosed fume hoods with exhaust systems. The gas is never intentionally released into open air.
Density as a Liquid
Fluorine becomes a liquid at extremely cold temperatures, boiling at −188.13 °C (−306.6 °F). At that boiling point, liquid fluorine has a density of about 1.513 g/cm³, which is noticeably denser than water. If you could somehow contain liquid fluorine in water (you can’t safely, since fluorine reacts violently with almost everything), it would sink.
The jump from gas to liquid represents roughly a 900-fold increase in density. That’s typical for gases condensing into liquids, but the specific number matters in cryogenic engineering, where liquid fluorine has been used as a rocket propellant oxidizer.
Density as a Solid
Cool fluorine even further and it freezes into a solid. The most studied form, called alpha-fluorine, is stable below about −227 °C (45.6 K). It forms a monoclinic crystal structure with four molecules packed into each unit cell. At 23 K (−250 °C), solid fluorine reaches a density of 1.97 g/cm³. Warm it slightly to 39 K (−234 °C) and the density drops to 1.93 g/cm³ as the crystal lattice expands.
Visually, solid fluorine is hard, brittle, and opaque with a faint yellowish tint. It’s not something encountered outside of specialized cryogenic research.
How Temperature Changes Fluorine’s Density
Like all substances, fluorine gets less dense as it warms up. Experimental measurements from the National Institute of Standards and Technology show this clearly across a range of temperatures. Near 83 K (−190 °C), just below the boiling point, liquid fluorine has a molar density around 31.7 mol/L, which translates to roughly 1.20 g/cm³. By 203 K (−70 °C), well above the boiling point and under pressure to keep it dense, the molar density drops to about 23.9 mol/L.
For the gas phase at normal pressure, density follows a simple relationship: it’s inversely proportional to temperature. Double the absolute temperature and the density halves. At 0 °C you get 1.696 g/L; at around 273 °C (546 K), you’d get roughly half that.
Quick Density Summary by Phase
- Gas (0 °C, 1 atm): 1.696 g/L
- Liquid (−188 °C): 1.513 g/cm³
- Solid (−250 °C): 1.97 g/cm³
These values place fluorine in a middle range among the halogens. Chlorine gas, for instance, is denser at 3.2 g/L because its molecules are heavier. Fluorine’s relatively low atomic weight keeps its density modest compared to its chemical cousins, even though it is still heavier than air.