Helium is actually lighter than hydrogen, not heavier. Helium is the second lightest element in the periodic table, while hydrogen is the lightest. The atomic mass of helium is approximately 4 atomic mass units (AMU), while the atomic mass of hydrogen is about 1 AMU.
The difference in atomic mass between helium and hydrogen arises from the composition of their respective atomic nuclei. In the case of helium, it consists of two protons and usually two neutrons, giving it an atomic mass of 4. Hydrogen, on the other hand, typically contains a single proton and no neutrons, resulting in an atomic mass of approximately 1.
The variation in atomic mass is primarily due to the difference in the number of subatomic particles (protons and neutrons) within the nucleus of each atom. Since helium has more particles in its nucleus, its atomic mass is greater than that of hydrogen.
However, it is important to note that when comparing the density of hydrogen and helium gases, hydrogen gas is lighter and less dense than helium gas. This is because hydrogen is a diatomic molecule (Hâ‚‚), while helium is a monatomic gas (He), and the presence of the extra hydrogen atom in each molecule increases its overall mass compared to helium.