Is helium lighter or heavier than air?

Helium is indeed lighter than air, which is what makes it an effective lifting gas. Air is primarily composed of nitrogen and oxygen, which have a higher molecular weight compared to helium. Helium has an atomic weight of about 4 grams per mole, whereas the average molecular weight of air is approximately 29 grams per mole. This significant difference in molecular weight allows helium to rise in the atmosphere, as gases will naturally move from areas of higher density to lower density.

This property is the reason why helium is commonly used in balloons; they ascend when filled with helium due to being less dense than surrounding air. Understanding this concept is particularly important in various applications, including the welding and fabrication industry, where different gases are used for specific functions based on their properties, including weight and buoyancy.