Eli5 what controls if a gas is heavy or not? Example helium Seems to defy gravity while something like xenon I assume would sink
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Gas is actually really simple to figure this out for. See, at the same temperature and pressure, all gases have about the same number of molecules in a space. An unpressurized 2L bottle of oxygen will have the same number of molecules as a 2L of xenon or helium or air.
So it only depends on how heavy each molecule is, and that you can figure out from the periodic table.
To see if it will float, you just need to know if it’s lighter than air, so let’s see how heavy a molecule of air is. Air is made up of nitrogen and oxygen, both of which exist in molecules of two atoms, so N2 and O2. Nitrogen’s weight on the table is 14 and oxygen’s is 16, so the molecule weights are 28 and 32. Let’s call it 30 for air as a whole, to save some math.
Helium comes in single atoms, and its weight (per the table) is 4, so it’s 4/30 as heavy as air. Hydrogen’s weight is 1, but it also comes in molecules of 2 atoms so its weight is 2.
Xenon, and all noble gases, come in single atoms, and xenon’s weight is 131. *Way* heavier than air.
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