So why does sulfur hexafluoride behave like a liquid? It all comes down to weight and density, or how close the molecules inside a substance are to each other.
Sulfur hexafluoride is a particularly dense gas -- almost five times as dense as air. This just means that more of its molecules inhabit a given space than with most other gases. On top of this, each molecule of SF6 also weighs more than an air molecule. If you released a balloon full of sulfur hexafluoride (molecular weight 146) and a balloon full of helium (molecular weight 4) into the air (molecular weight 29), the helium balloon would rise to the ceiling and the one filled with SF6 would sink to the floor.
