Helium is the second most abundant element in the universe. Most of it was created in the seconds after the Big Bang and is found in stars. Obviously that’s not exactly accessible, so where do we find the helium used to inflate birthday balloons?
The simple answer is that it’s brought up as a gas from deep underground, but the story of how it came to be there is an interesting one.
First, we need to understand a little about the structure of helium. The helium atom is extremely small, only hydrogen is smaller, and consists of a nucleus of 2 protons surrounded by 2 electrons. Being very small, it’s also light, so light in fact that there is very little helium in our atmosphere. It simply drifts away. This atomic structure also means that helium exists only in liquid and gaseous forms. There is no such thing as solid helium.
So how does it get to be underground?
Helium is created from radioactive decay. Radioactive decay means that over time a material throws off some of the protons that it’s made from. These tiny particles are simply ejected from the surface much as fat will spit in a frying pan. When uranium decays the particles thrown off consist of two protons, surrounded by two electrons and are known as alpha particles. But as we know, helium consists of two protons and two electrons, so helium and alpha particles are one and the same.
Uranium is found in extremely old igneous rocks (those created by volcanic activity rather than by sedimentation,) so regions with a lot of deep granite can also be helium producing. However, in most places the helium atoms, being extremely small, simply diffuse through the rock above, and go into the atmosphere. But in a few parts of the world, principally the central USA, Algeria and possibly Siberia, the igneous rock is covered by a layer of decomposed plant material, which produces natural gas and oil, and then capped with an impermeable material such as clay.
The clay cap traps natural gas, and helium emitted from uranium is caught in the same place. This is why, in just a few parts of the world, helium is extracted along with natural gas. Unfortunately though, helium is usually present only at low concentrations, so it has to be separated out. This is done by first liquefying the raw, extracted gas, and then using ‘fractional distillation’ to, in effect, boil off the lighter constituents.
While new helium is no doubt still being created deep within the earth, this happens very slowly and there are few places where the gas can be captured. This means we have only a finite supply of helium and when it has all been extracted there will be no more.
