Why after a week a helium bloon is deflated?

Answer 1

Helium is a very small molecule. In a balloon it is under relatively high pressure and pushes against the rubber skin to inflate the balloon.

To us the rubber skin of a balloon looks quite solid with no holes (Holes in balloons are generally looked at as bad things!). However the balloon skin is riddled with very small holes and gaps between the rubber molecules. The helium molecules are eventually forced through these gaps by the pressure until they escape to the outside air. As they escape the balloon deflates.

Think of inflating three balloon: One with ordinary air, one with helium and one with hydrogen (a molecule even smaller than helium)

The hydrogen ballon would deflate first as the very small molecules pass through the rubber very quickly. The helium next. Finally the air would slowly leak out. This suggests that the holes in the rubber are more the size of air molecules than helium molecules.