Although being the second most abundant element in the universe, making for up to 23 percent of all ordinary matter, helium is one of the most scarce elements on Earth, having a concentration averaging about 5.2 parts per million in Earth's atmosphere. Some studies even suggested that within a few years, the helium reserves on the planet will be completely depleted, leading to a crisis that is most likely to affect the several segments in science for which the use of helium is critical.
But why is there so little helium on Earth and where does it come from? Helium is the second lightest element in the universe, meaning that it is also lighter than the air in our planet's atmosphere, which largely consists of a mix of nitrogen and oxygen. This enables helium to escape into the upper layers of the atmosphere and eventually into space. Another reason for the low concentrations of this element on Earth is that helium is a noble gas.
Basically, all noble gases have extremely low or no chemical reactivity. Helium for example, is completely unreactive and so far no naturally occurring compounds have been found, unlike other elements such as hydrogen, oxygen, iron, etc., which tend to form bonds with other elements resulting in heavier than air substances. Some unstable compounds have been created nonetheless during electric glow discharges, such as HeNe, HgHe10 or He2 and HeH.
As earlier said, helium occurs naturally in the universe in vast quantities, though on Earth it is relatively scarce. It is usually found in natural gas pockets in concentrations as high as 7 percent. When the planet formed, some 4.6 billion years ago, virtually no helium was present. Currently however, some 3,000 tones of this element are generated by Earth's crust every year.
This is because helium is produced on Earth through the radioactive decay of heavy elements such as uranium and thorium. These two generally decay through alpha particles, which are practically helium nuclei that then combine with electrons from the medium to form atoms that afterwards either escape into the atmosphere or gather into pockets along with natural gases.
Once natural gas is extracted, helium is obtained through a fractional distillation process to purities of up to 99,995 percent, the remaining 0,005 percent representing neon impurities. Due to its low weight, helium has a great deal of applications involving lighter than air aircrafts. It also has the lowest boiling point, meaning that it is generally used as a means to bring metals to temperatures close to absolute zero.