Let us use the periodic table, which indicates the relative atomic masses of chemical elements. Hence, the relative atomic mass of hydrogen (Mr) is 1, whereas for helium Mr = 4. Helium is an inert monatomic gas, so M (He) = 10-3*Mr (He) kg/mol = 10-3*4 kg/mol = 0,004 kg/mol. Where do we begin when we are to speak about helium? Right, with the molar mass of helium. It is a well-known fact that this inert gas is the most stubborn of all gases. Its atoms in no circumstances do not want to engage with one another or with atoms of other elements. Exactly the obstinate unwillingness of helium atoms to interact explains why this gas was the last, which yielded to chemists’ efforts and succumbed to liquefaction (helium has the known lowest critical temperature T = 5.25 K).