Answer:
Because the molecules are not packed together as much than the air, in a fixed volume of helium there are fewer particles than in the same fixed volume of air.
This means that the mass of the fixed volume of helium is smaller than the one of air, and then the density, defined as mass/volume, is also smaller.
This is why the balloon floats, because the air is denser, it tries to go down, pushing the ballon upside.
The case is similar for a piece of metal in water, as the metal is way denser than the water, the metal will sink, but for something not as dense, like a balloon with air, the balloon will remain in the surface of the water.
