# why hydrogen is so light?

I was studying about properties and behaviour of hydrogen and found that hydrogen is lightest element.It is so light that 1 litre of hydrogen at N.T.P weighs only 0.0980 grams.

The first statement that hydrogen is lightest element seems reasonable to me,but the data 1 litre of hydrogen at N.T.P weighs only 0.0980 grams has shocked me. Isn't it so so so light.

Hence,the question i m asking is why hydrogen is so so so light??

The reason behind it is not given in my book so i supposed to ask it here.I shall be very thankful iif someone of you give me a reasonable answer for it.

• Hydrogen is the element with only one proton in the nucleus. The primary isotope has no neutrons (deuterium has one neutron, tritium has two). So, you can't get any simpler than one proton and one electron hence it is the lightest element. – Jon Custer Oct 21 '16 at 13:16
• Hydrogen is so light because some element has got to be the lightest of them all. Why are small kids so small? – Ivan Neretin Oct 21 '16 at 13:43
• And one litre of helium only weighs ~0.196 grams. So? – Jan Oct 21 '16 at 17:47
• Was it for me @Jan – Vidyanshu Mishra Oct 21 '16 at 19:07
• Yes it was. Gases are relatively light (at least most of them), that’s why they’re gases. Granted, helium and hydrogen are even lighter, but not all too exceptionally. Finally, consider (positive) muonic hydrogen which could weigh as little as I think a quarter of hydrogen (but isn’t stable). – Jan Oct 21 '16 at 22:31

# In short

Hydrogen is so light because its molar mass is only $2\ \mathrm{g\ mol^{-1}}$ and a fixed amount of gas occupies a fixed volume regardless of the species of the gas. Hydrogen is the lightest element.

# In long

By the Ideal gas law:

$pV=nRT$

where $p$ is pressure, $V$ is volume, $n$ is amount, $R$ is a constant, $T$ is temperature.

The significance of this law is that a fixed amount of gas occupies a fixed amount of volume (in a sealed container), regardless of the species of the gas, as long as the pressure, the volume, and the temperature are constant.

Applying s.t.p (standard temperature and pressure):

$(101325~\mathrm{Pa}) (0.001~\mathrm{m^3})=n(8.3144598~\mathrm{J\ mol^{−1}\ K^{−1}})(298.15~\mathrm{K})$

$n=0.040874$

$m=n(2~\mathrm{g\ mol^{-1}}) = 0.0980~\mathrm{g}$

A species of chemical (yes, everything is a chemical) cannot be light by itself. The weight (and the mass) of a chemical depends on the amount of the chemical. You probably meant not dense.

• To make it a bit longer you should use a Peng-Robinson or Soave-Redlich-Kwong EOS. :p – Hexacoordinate-C Oct 21 '18 at 1:50
• 101325 Pa and 298.15 K is not STP. – Loong Jun 15 at 11:42

Because the dihydrogen molecule is the least massive gas molecule under normal conditions

The key thing about gases is that a given amount of gas always takes up the same volume regardless of the makeup of the gas molecule (this is an approximation as not all gases are ideal gases, but it is usually a good approximation).

So one mole of any gas takes up about 22 litres at standard temperature and pressure. Because of this the only thing that matters for the density is how heavy the gas molecule is. H2 has an atomic mass of about 2, the next nearest common gas is helium which has a mass of 4 and is therefore twice as dense. Go to the other end of the scale with xenon (mass about 131) and you have something which is about 65 times denser than H2.

This and the fact that there are no elements with fewer protons than hydrogen, explains the low density of hydrogen gas.