There are two problems with this question, one is that inert gas is more than one substance and the second is that so is air. Air is a mixture of nitrogen, oxygen, carbon dioxide, argon and other gases. The best way to determine its mass is to take an average between these components.
Next, what's an inert gas? Strictly speaking, the inert gases are the noble gases, which are the elements in group 0 of the Periodic Table. They are helium, neon, krypton, radon, xenon and argon. Hang on..., isn't argon in air?
Helium is the lightest inert gas and as anyone who has seen a helium balloon can testify, it is lighter than air. The further down group 0 you go, towards the likes of xenon and radon, the heavier they get. From krypton onwards, they are all heavier than air except....
... except that all of the noble gases, or the inert gases, can be found in air, in very small amounts. So in a weird way, if you're asking if inert gases are heavier than air, you're asking if they're heavier than themselves.
But to put it in the simplest terms, there is more than one inert gas. Helium can be considered an example of an inert gas that is lighter than air, whereas xenon can be considered an example of one that is heavier.
100% of hydrogen is lighter than air
Hot air, hydrogen or helium are lighter than cold air.
Hydrogen and Helium are both lighter than air.
Greenhouse gasses are NOT "obviously lighter than air"; carbon dioxide is significantly heavier than air.One way to weigh something that's lighter than air is to weigh it in a vacuum.
HELIUM is not toxic it is lighter than air.
The balloon is lighter than air.
100% of hydrogen is lighter than air
Yes methane is lighter than air because it is less dense than air.
No Argon is not lighter than air. Argon is 25% more dense than air.
blimp or hot air balloons are lighter than air crafts
Hot air, hydrogen or helium are lighter than cold air.
Hot air, hydrogen or helium are lighter than cold air.
"Lighter than air." Think about it for a second.
Hydrogen and Helium are both lighter than air.
Natural gas is lighter than air and rises.
HYDROGN IS 14 TIMES LIGHTER THAN AIR.
Just, simply, Lighter than Air Vehicles are lighter than air, so that makes them fly, and Heavier than air vehicles are heavier than air, but they use airfoils on the wings to fly.