What is the heaviest gas?
Tungsten hexafluoride
Tungsten(VI) fluoride, also known as tungsten hexafluoride, is an inorganic compound with the formula WF6. It is a toxic, corrosive, colorless gas, with a density of about 13 g/L (roughly 11 times heavier than air.[1][2][3])  It is one of the densest known gases under standard conditions.[4] WF6 is commonly used by the semiconductor industry to form tungsten films, through the process of chemical vapor deposition. This layer serves as a low-resistivity metallic "interconnect".[5] It is one of seventeen known binary hexafluorides.