There is twelve times as much helium in the universe as all the other elements (except hydrogen) combined, but it’s one of the rarest elements on Earth.
It is so light that it floats to the very top of our atmosphere and diffuses into space, so most of the helium we have on Earth (including every atom in a helium balloon) has recently come from the radioactive decay of uranium and other radioactive elements.
Luckily it’s completely safe, because one of helium’s superpowers is that it is inert. There’s practically nothing you can do to it to make it react with anything else.
Its second superpower is when it’s cooled to near absolute zero, it condenses into a liquid but never freezes into a solid.
This is a unique set of properties, so most of our uses for helium are not for balloons, but for a range of special tasks. Things that we need to cool to really, really low temperates to function like MRI scanners and particle accelerators are cooled by near-absolute zero liquid helium. This is called ‘cryogenics’.
When we’re manufacturing sensitive equipment like fiber optic cables and semiconductors, they tend to react with oxygen and other things in the atmosphere. So we make some factories airtight and fill them up with helium instead.
But it’s most well-known use is for balloons. In the same way that oil floats on water, helium balloons float on air. This happens because air is a mix of nitrogen, oxygen, hydrogen, carbon dioxide and other gasses, and helium is lighter than all of them except hydrogen.
When you let go of a helium balloon, it will deflate over a few days as the helium escapes through tiny holes in the balloon. The balloon will slowly settle back down to the ground, while the helium inside will rise towards and pass through the edge of space, and it will never to return to the Earth.
Helium balloons try to float ‘on top’ of air in the same way that oil floats on water.