Helium-filled balloons

D. Indumathi, The Institute of Mathematical Sciences, Chennai

We have all seen pictures of helium balloons floating away up in the sky. You may also have read of early explorers who rode the skies in hot air or helium-filled balloons before the invention of aeroplanes. How do these balloons work? What is so special about helium? And where does it come from?

Why helium balloons rise so high
Helium balloons work by the law of buoyancy. It's based on the same principle as why you float on water. In fact, any body that displaces less weight of water than its own weight, ends up floating on water. While a human body barely floats, a steel plate sinks while an empty steel cup (containing steel and air) floats for the same reason.
Helium is lighter than air. Helium weighs 0.1785 grams per litre while Nitrogen (which air is mostly made of) weighs 1.2506 grams per litre. The difference is 1.25-0.18=1.07 grams, that is roughly 1 gram per litre. This means that a 1-litre bottle of air (that is, an "empty" 1-litre bottle!) would weigh just 1 gram more than a similar bottle of helium. This doesn't seem like much but it all adds up.
One way of understanding the law of buoyancy is to look at the effect of gravity. All masses are attracted to the Earth due to gravity, and the force of attraction is proportional to the mass of the body. So the heavier substance is attracted more and the lighter substance is attracted less. So if you have a helium filled balloon, it is attracted less than a similar amount of air to Earth. In effect, it begins to "float" and rises, pushing an equal amount of air downwards. As it rises, it sees a thinner and thinner atmosphere. It keeps rising until the density of air outside equals the density of the helium, at which point it floats in place for ever. Actually, to be more precise, it will rise until the total weight of the helium and the balloon material is less than the weight of the air it displaces. That is why balloons are made of light materials like silk.

BOX How balloons were used for space flights
Since the difference in weight between helium and air is just one gram per litre, it needs large volumes of helium to lift heavy weights such as men and machines. In order to achieve lift, the total weight of helium as well as the machines must be less than that of the displaced air. Consider the balloon to be spherical. Then we know its volume is (4/3) pi r^3 where r is the radius of the balloon. For a 10 metre balloon with radius 5 m, the volume is (4/3)x (pi) x (5x5x5) which is about 500 m^3. Since 1 m^3 = 1000 l, this is about 500,000 litres. Since the effective lifting force is 1 gm per litre, such a balloon can lift a cargo of 500,000 gm or 500 kg. NASA and other space and weather organisations send many such balloons into space to understand the composition of the upper atmosphere. Hydrogen gas weighs even less than helium, but it is highly flammable.
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Discovery of helium
Helium is one of the most abundant elements in the universe, which is made of about 75% hydrogen and 25% helium. Most of it was produced when the Universe began with the Big Bang, but some of it is still being produced in stars. However, most of it exists outside of Earth and its atmosphere. Helium was discovered in 1868, independently by French astronomer Pierre Janssen and English astronomer Sir Joseph Lockyer. They were studying an eclipse of the Sun using spectrometers.
A spectrometer, as the name suggests, separates light into different bands of colour using a prism. For instance, sunlight is separated into the entire rainbow of VIBGYOR colours by a prism or a spectrometer. However, some light sources contain only certain colours, depending on the atoms or molecules that are present in it. For instance, mercury vapour lamps contain all colours but not a continuous spectrum (the light in a spectrometer from mercury vapour shows bands of colour) while sodium vapour lamps have only yellow light. The colours seen in the spectrum are used to identify the atoms or elements in the gas.
Both scientists observed a band of yellow light from the Sun that could not be identified with any known element. In fact, it was close to the two lines from sodium light, but not exactly the same. See figure. News of their findings reached the scientific world on the same day, and both men are generally credited with the discovery. Janssen actually travelled to Guntur in India to make his observations.
Lockyer suggested the name helium for the new element, derived from the Greek word helios for the Sun. Today we know that hydrogen burns to helium in the hot core of the Sun, producing the sunlight that is responsible for all life on Earth.

A Noble gas
Helium has two electrons orbiting around a central nucleus containing two protons and two neutrons. Since only two electrons can fit in the first electronic shell, the shell is complete or ``closed'' and so helium rarely interacts with any other gas or metal. Hence it is called a rare or Noble gas.
The only gas lighter than helium is hydrogen, which does not occur naturally on Earth. In fact, our atmosphere is made up of nitrogen, oxygen, and argon (also a rare or noble gas). Gases such as carbon-dioxide, water vapour and some other gases such as methane, nitrous oxide, ozone and other greenhouse gases, are present in small quantities. All these gases are heavier than helium. See figure. Here helium, neon, argon, krypton are rare gases, the lightest of which are present in trace amounts in the upper atmosphere or exosphere.

How helium is made in Earth
There is no chemical way of making helium and all the helium that we use has been accumulating in the rocks from decays of radioactive substances such as Thorium and Uranium. See Box. There is hardly any helium in the air. In places that have a lot of uranium ore, natural gas (used for cooking, etc) tends to contain high concentrations of helium (up to 7 percent). The decay of uranium emits lots of alpha particles and alpha particles are just helium nuclei. They can easily acquire two electrons and become helium atoms. The gas (helium and the natural gas) collects in pockets in the underground rock, almost like it is a sealed container. Once the rocks are tapped for their natural gas, the helium gas comes out as well, and will be lost to space if it is not bottled and stored at once. It is separated (cryogenically distilled) out of natural gas to produce the helium we put in balloons.

BOX How radioactivity produces helium
Uranium and Thorium are two of the most common radioactive elements on Earth. Radioactive means that, even left to themselves, they decay or transform into other elements. They do this mostly by emitting particles called alpha and beta particles. Alpha particles are helium nuclei, while beta particles are electrons. These exotic names were given when it was not yet known what these emitted particles were. Very often even the "daughter" nuclei, that is, the element into which uranium and thorium decay, are not stable, and they decay as well. This establishes a "decay chain", with a series of elements being formed, one after another, by emissions of either alpha or beta particles. The chain of decays stops when the final nucleus is stable. This is most often the element lead. There are three series of decay chains where helium is produced in the Earth.

The "thorium series" begins with thorium-232. The number 232 indicates the atomic mass of thorium. Beginning with naturally occurring thorium-232, this series includes the following elements: actinium, bismuth, lead, polonium, radium, radon and thallium. The series terminates with lead-208.
The "actinium series" begins with the naturally-occurring isotope of uranium, U-235. This decay series includes the following elements: actinium, astatine, bismuth, francium, lead, polonium, protactinium, radium, radon, thallium, and thorium. This series terminates with the stable isotope lead-207.
The "uranium series" begins with naturally occurring uranium-238. It includes the following elements: astatine, bismuth, lead, polonium, protactinium, radium, radon, thallium, and thorium. The series terminates with lead-206.
All the decay chains are shown in the figures. The places marked alpha are where helium is produced in the decay.
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Other uses of helium
Helium is not just used to fill balloons. It is used in the liquid form in MRIs to cool the magnets because it has a very low boiling point of 4.222 K (−268.928 °C). So it can be used to cool metals down to very low temperatures. This is required for producing superconductivity, which is a state reached by the metal when it has zero electrical resistance (and hence becomes a perfect conductor).
Just think about it: all metals conduct electricity and become hot. You may have noticed this with a light bulb or fan. This means that some of the energy (electricity) you are supplying is being wasted in the form of heat. This is because of the resistance of the metal or wire, which dissipates the energy in the form of heat.
Now, if the metal is cooled below its superconducting transition temperature, no energy is dissipated as heat and the current circulates practically for ever with no loss. Of course, energy is needed to cool the metal below this temperature but after that there are no losses. This is very important in electromagnets where huge currents are required to generate large magnetic fields leading to huge losses due to unwanted heat generation. If the magnets are made out of superconducting material (not all metals are superconductors), they will become cheaper to operate. Hence machines such as MRIs which generate very large magnetic fields (7 T or more) usually use superconducting magnets which are cooled using liquid helium.
A large amount of helium is used to cool the superconducting magnets in the Large Hadron Collider (LHC) in Geneva where the Higgs boson particle was discovered in 2012. The LHC uses 96 tons of liquid helium to maintain the temperature at 1.9 K!
Helium is also used in the manufacture of LCDs and fibre optics. It is used in airships, to cool nuclear reactors and infra red detectors, satellite and spacecrafts, and solar telescopes. NASA also uses it to clean fuel from its rockets. The Saturn V rocket used in the Apollo program used about 370 million litres of helium for its launch. While helium is so cheap today that it is commonly used to fill birthday balloons, it is clear that these many uses of helium will soon cause a global shortage. While most of the gas is now found in mines in the United States, Iran and Qatar, other countries such as Tanzania have begun looking for this gas as well.

Image Credit: PVerhage - Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=3844368; also wikipedia