Question

how is helium formed

Answer 1

The helium that is in the atmosphere comes from alpha particles emitted by radioactive decay (see How Nuclear Radiation Works for details on alpha decay). In places that have a lot of uranium ore, natural gastends to contain high concentrations of helium (up to 7 percent). This makes sense, since the decay of uranium emits lots of alpha particles and a natural gas pocket tends to be a sealed container underground.Helium is cryogenically distilled out of natural gas to produce the helium we put in balloons.

Answer 2

The helium is formed during the natural radioactive decay of elements such as uranium and thorium. These heavy elements were formed before the earth but they are not stable and very slowly, they decay. One mode of decay for uranium is to emit an alpha-particle.