Or maybe….steel is heavier than cork… WRONG!!
It depends how much you have!….a steel sewing needle doesn't weigh as much as a car sized piece of cork! And a roomfull of hydrogen is heavier than a thimblefull of steel…..
The important word is DENSITY. Hydrogen is less dense than air, steel is more dense than cork. Density compares the mass of different substances for the SAME volume! Density tells us how many kilograms there are for one cubic metre