Eli5 what controls if a gas is heavy or not? Example helium Seems to defy gravity while something like xenon I assume would sink

Question

Eli5 what controls if a gas is heavy or not? Example helium Seems to defy gravity while something like xenon I assume would sink

In: 1

Answer 1

The heaviness of a gas is its density, which has a correlation with its molecular mass as a general rule.

Helium doesn’t defy gravity, it’s just floating on air. Air and Helium are both fluids.