Helium (₂He⁴) is the second most common chemical element in the universe and also the second lightest. It is a Noble gas (i.e: It doesn’t combine with other atoms, or even with itself above a fraction of a Kelvin). Young Earth believers use it in 2 mutually contradictory ways to try to prove that the Earth is only a few thousand years old.

1. First, they say that substantially all the Helium on Earth is produced by radioactive decay (beta particles are Helium nuclei). So far, so good (if we ignore the prevalance of Helium in the solar system). And that the planet is full of decaying isotopes. Good, good. Now, if this is being produced at a given rate (some not unlikely number is cited here), then there should be much more in the atmosphere if it is millions, much less billions, of years old, because there is no mechanism (such as combining with other chemicals) to remove it.
   Okay, here we have 2 problems.
1. If there is no way for Helium to escape, then the atmosphere must be mostly Helium because of its commonness in creation in general.
2. Helium is light and not sticky; it is the most likely element to be booted from the top of the atmosphere (to which it naturally rises) by collisions with higher energy particles (like solar wind, cosmic rays) or to simply evaporate into space from thermal excitation. That’s both why there is more nitrogen than helium, and why the helium doesn’t accumulate.
2. Secondly, given that Helium is produced in the Earth’s crust by decay and is injected into fluorite grains (one of the common crystals used to date rocks), then it should have had plenty of time to escape. There is detectable helium in these grains, and therefore the world must be young.
1. Helium is continually being injected by decay processes from within and without these grains. It wasn’t all put in at once, so some was injected recently.
2. Although Helium does not chemically stick to anything, it can easily be trapped in the potential wells between the atoms of a crystal. Even at high temperatures, Helium is as likely to migrate into these traps as out of them, so the amount of Helium in these crystals should be proportional to the amount of beta-decay isotopes in the matrix around them in a steady-state calculation (so that it doesn’t matter how old the rock is). It works out.

So, pick your “theory”: Either Helium is so very light and slippery that it can get out of crystals encapsulated by other materials that are just as helium rich; or it is heavy and or sticky and cannot escape the atmosphere, and wasn’t there in the first place, either.