Helium

Helium is the second lightest element in the universe and the only one discovered in space before it was found on Earth. It is a colourless, odourless gas that makes balloons float and voices go squeaky, and deep inside stars, it is the ash left behind after hydrogen is burned up by nuclear fusion.

• Atomic Number22 protons, 2 electrons
• Atomic Mass4.00260 u4× heavier than hydrogen
• State at Room TempGasballoons float in it
• Density0.0001785 g/cm³7× lighter than air
• Melting / Boiling-272.2°C / -268.9°Ccoldest liquid on Earth
• Discovered1868Janssen & Lockyer, 1868

What is helium?

Helium is a noble gas, which means it almost never reacts with other elements. Its atoms have two protons, two neutrons and two electrons, and that tightly packed arrangement makes helium incredibly stable. It will not burn, will not rust, and will not join with any other substance under normal conditions. Of all the noble gases, helium is the second lightest and the most inert.

Helium gets its name from Helios, the ancient Greek god of the Sun. Scientists chose the name because helium was first spotted in the light coming from the Sun in 1868, nearly three decades before anyone found it on Earth. The yellow spectral line it left in sunlight matched no known element, pointing to something entirely new.

Where you find helium

In space

Stars like our Sun are enormous balls of hydrogen being turned into helium by nuclear fusion. Deep inside the Sun, hydrogen atoms slam together so hard that they fuse into helium, releasing the flood of light and heat that warms the Earth. The Sun is approx. 24% helium by mass. The giant planets Jupiter and Saturn are also wrapped in thick layers of helium mixed with hydrogen, and Uranus and Neptune contain helium too.

On Earth

On Earth, almost no helium exists as a free gas in the open air. Because it is so light, any helium that escapes into the atmosphere quickly drifts up and into space. Instead, helium is found trapped underground in natural gas deposits, produced by the slow radioactive decay of uranium and thorium deep in the crust.

• Underground gas wells. Helium is found dissolved in natural gas, especially in large fields beneath the United States, Qatar, Algeria and Russia. Most commercial helium comes from these wells.
• Radioactive decay. When uranium and thorium decay inside rocks, they release helium atoms (alpha particles). Over millions of years these accumulate in underground traps: the source of all Earth's helium.

How we use helium

Here are the most important ways people use helium today:

• MRI scanners. Hospitals cool the powerful magnets inside MRI scanners using liquid helium, which is the coldest liquid in the world, just four degrees above absolute zero. Without helium, MRI machines could not function.
• Balloons and airships. Helium lifts balloons because it is lighter than air, and unlike hydrogen it cannot catch fire, making it far safer for both party balloons and large airships.
• Welding. Helium is used as a shielding gas to stop hot metal reacting with oxygen in the air while it is being welded together.
• Semiconductor manufacturing. Ultra-pure helium is used to cool and protect silicon wafers during computer chip production.

How it was discovered

Helium is the only element first discovered off Earth. During a total solar eclipse in 1868, the French astronomer Pierre Janssen spotted a bright yellow spectral line in sunlight that matched no known element. The English astronomer Norman Lockyer studied the same line independently and proposed a brand-new element, naming it helios after the Greek sun god.

It took another 27 years before William Ramsay isolated helium on Earth in 1895. He was analysing gases released from the mineral uraninite and noticed a spectral line identical to the one seen in sunlight. He had found the same element, trapped inside a radioactive rock all along. The gas had been hiding in plain sight for nearly three decades.

Helium is the lightest noble gas and the first of a group of elements that almost never react with anything. The next element along, with three protons, is lithium, a soft metal that behaves in a completely different way.