Helium is a chemical element. Its official symbol is He, and its atomic number is 2, which means it has two protons in its nucleus. Helium was named after Helios – the Greek Titan of the Sun – because it was first observed when analysing sunlight during a solar eclipse in 1868.
Helium is one of the seven elements known as noble gases. Noble gases have the exact amount of electrons to completely fill the space around their atomic nucleus.
In older models, electrons are shown in shells. Helium atoms have two electrons in their outer shell and the other noble gases have eight electrons in their outer shell. This is the maximum number that will ‘fit’ each shell, making the atoms stable.
The current model depicts the electrons moving in space around the nucleus in a more cloud-like formation and not set in fixed shells. As a result, noble gases are stable. They don’t need to react or join up with other elements to form compounds, nor do they want to give up their electrons, so helium and other noble gases like neon and argon tend to float around by themselves.
The noble gases are located in the far right column of the periodic table of elements – in group (column) 18.
There are stories about how the term ‘noble gases’ came about. The term may have come from noble metals – gold, silver and copper. These metals are not as reactive as other metals, and in the old days, only aristocratic (noble) people could afford objects made from them. Another story says that noble gases stay by themselves and don’t mix with other gases – just like nobles who preferred not to mix with common people. Fortunately, the chemistry of noble gases is more precise than their etymology (word origin)!
An abundant yet rare element
Helium is the second most abundant element in the universe. It makes up about 24% of the elemental mass. Most of this helium was formed within minutes of the Big Bang. New helium atoms are created in stars. Stars like our Sun have extreme temperatures that fuse hydrogen atoms together to form helium atoms.
On Earth, it is a different story. Helium is actually quite rare. There is very little of it in Earth’s atmosphere – only 0.0005% by volume. Because it is so light, helium does not stay in the atmosphere – it escapes into outer space! There are helium reserves underground. Terrestrial helium is created by radioactive decay and becomes trapped with other natural gases. When the gases are extracted from the ground, the helium is separated from the rest of the gases.
Uses for helium
During the first half of the 20th century, helium was quite scarce. The United States had significant reserves but banned exports to other countries. The US military used it to create helium-filled airships during World Wars I and II. Helium is a stable gas, so it is much safer to use than hydrogen, which is reactive and highly flammable. The export ban meant other airships such as the Hindenburg were forced to use hydrogen as the lifting gas. Commercial airship travel ceased after the Hindenburg caught fire in mid-air.
At low temperatures, helium becomes a liquid and is very useful as a coolant. Liquid helium cools the magnetic coils of MRI scanners in hospitals, the electromagnets in the Large Hadron Collider, satellite instruments and rocket fuel. Deep-sea divers replace the nitrogen in their air tanks with helium to protect themselves from ‘the bends’.
We encounter helium quite often in our daily lives – in party balloons and in some of the lasers used to scan merchandise in grocery and retail stores. One time we hope to avoid using helium is when we are in the car – it fills the airbags in the event of a crash.
When helium is cooled to near absolute zero, it becomes a superfluid – a state of matter unique to liquid helium. The superfluid has an apparently frictionless flow. It can also move along a surface, travelling upwards against the force of gravity.
The speed of sound in helium is about three times the speed of sound in the air. This is why the human voice sounds high pitched when a person breathes in helium. Although helium gas is non-toxic, it displaces oxygen needed for respiration. Breathing helium from a balloon is not safe and should not be done.
Find out where helium atoms come from in the article How elements are formed. The article includes the interactive Universal element formation, which shows the stages of star life cycles and how elements are formed deep within their cores.
Find out how MRI scanners work and how liquid helium keeps the temperature at a chilly -269°C in the article Magnetic resonance imaging (MRI).
Find out what happens to helium formed in the universe in this Ask an astronomer post.