Question

Explain why the number of atoms in one mole of hydrogen gas is double the number of atoms in one mole of helium gas?

Answer 1

Avogadro’s law states that equal volumes of gases at the same temperature and pressure will have the same number of particles or molecules. You should know that; a mole is a measure of $6.023\times {{10}^{23}}$ particles (which may be an atom or molecule or any substance that is dependent upon the gas).

Complete step by step answer:
Let us first have some knowledge on the mole concept.
So, mole is the unit of measurement in the International System of units (S.I.) for the amount of substance. One mole is considered as just a number whose value is equal to the Avogadro’s number i.e. $6.023\times {{10}^{23}}$ particles.
One mole in quantity implies that the matter under consideration contains exactly $6.023\times {{10}^{23}}$ number of particles (may be atoms, molecules, ions, electrons or any other elementary entities). So, we can say that one atom will have one mole.

Now, coming to the question given. Hydrogen is a diatomic element and therefore it will contain two atoms while helium is a monatomic element and thus it will contain one atom only. Two atoms of hydrogen mean, it will have one number of moles and as helium is monoatomic. So, it will have only one mole. We can see that the number of hydrogen atoms is just double that of helium atoms.
Hence, the number of atoms in one mole of hydrogen gas is double the number of atoms in one mole of helium gas due to its nature of existence in a gaseous state as a diatomic molecule.

Note: It is important to keep in mind how an element exists in a gaseous state. Hydrogen atoms always exist as a diatomic molecule in gaseous state and thus it has two atoms. Two atoms of hydrogen atom will equal to one mole while helium is a monatomic and is considered as an inert gas.