The helium used to fill birthday balloons doesn’t come out of the air, but from out of the ground. some of the gas might escape from the ground into the
atmosphere, but the majority of it is trapped in earth’s crust. based on what you have learned in today’s lab, give a reasonable explanation for the presence of helium gas in earth’s crust.

Helium is trapped in earth's crust mainly as a by-product of decay chain cycle of radioactive isotopes including uranium-238, uranium-235 and thorium-232. these radioactive isotopes continually decay  to  emit radioactive particles including alpha, beta and gamma  rays. when an alpha particle, each containing two protons and two neutrons, collide with two electrons, they form a helium atom. since earth's crust contains these radioactive particles, therefore much of helium is produced within earth's crust and gets trapped in it. 

On the galapagos islands charles studied finches