Helium doesn't actually defy gravity. It's more accurate to say that helium behaves differently under the influence of gravity than heavier gases like oxygen and nitrogen.

Here's why:

* Density: Helium is the lightest element, meaning it has a very low density. This means that a given volume of helium has a much lower mass than the same volume of air.

* Buoyancy: An object submerged in a fluid experiences an upward force called buoyancy. This force is equal to the weight of the fluid displaced by the object. Because helium is less dense than air, a balloon filled with helium displaces a volume of air that weighs more than the helium itself. This results in an upward buoyant force greater than the downward force of gravity on the balloon, causing it to rise.

In summary: Helium doesn't defy gravity, but its low density and the principle of buoyancy allow helium-filled objects to rise in air, giving the appearance of defying gravity.