Helium is actually lighter than hydrogen, not heavier. The confusion might arise from the fact that helium has a higher atomic number than hydrogen, which refers to the number of protons in an atom's nucleus. However, when we talk about the relative weight or density of gases, helium is lighter than hydrogen.
The atomic weight or mass of an element is determined by the combined mass of its protons, neutrons, and electrons. Helium has an atomic weight of approximately 4 atomic mass units (AMU), while hydrogen has an atomic weight of approximately 1 AMU.
The difference in atomic weight between helium and hydrogen can be attributed to the number of protons, neutrons, and electrons in their atoms. Helium has two protons and two neutrons in its nucleus, while hydrogen has only one proton and no neutrons. Electrons, which orbit the nucleus, contribute very little to the overall atomic weight.
Since helium has a higher atomic weight than hydrogen, it might be tempting to think that helium is heavier. However, when we compare equal volumes of helium and hydrogen gas, helium is significantly lighter. This difference in density is due to the lower mass of a helium atom compared to a hydrogen atom.
In practical terms, the lighter nature of helium compared to hydrogen is why helium is commonly used as a lifting gas in balloons and airships. Its lower density allows balloons to float because the buoyant force exerted on the balloon is greater than the weight of the displaced air. Hydrogen, despite being even lighter than helium, is less commonly used due to its flammability and safety concerns.