[LT4]

Why is hydrogen less dense then helium?  I know it is less dense, but I'm not sure why.  Could someone please explain it to me?

[jdurg]

What is density?  It's the mass of something divided by the volume it takes up.  So if something has a density of 22 grams per milliliter, then it means that in one milliliter of volume there are 22 grams of the substance.  

Now what else do we know about gases?  We know that one mole of a gas occupies 22.4 Liters of space at standard temperatures and pressures.  It doesn't matter what the formula of the gas is.  One mole of it will take up 22.4 liters of space.

So knowing that, you can see why hydrogen is less dense than helium just by looking at the periodic table.  

[Donaldson Tan]

i presume you are comparing hydrogen and helium in gaseous state. under the same pressure and temperature, both gases exhibit the same molar volume. however, 1 mole of Helium contains 4g whereas 1mole of hydrogen gas (H2) contains 2g. Given density is mass per unit volume, we can see that 1mole of helium contains more mass than 1 mole of hydrogen gas, therefore hydrogen gas is less dense than helium.