Helium

hélium

Helium

elio

helio

Helium

Discovered : by Sir William Ramsay in London, and independently by P.T. Cleve and N.A. Langlet in Uppsala, Sweden in 1895

Helium Name Origin : The name is derived from the Greek ‘helios’, sun

Elemental helium is a colourless odourless monoatomic gas. Helium is the second most abundant element in the universe after hydrogen. a particles are doubly ionised helium atoms, He2+. Helium is used in lighter than air balloons and while heavier than hydrogen, is far safer since helium does not burn. Speaking after breathing an atmosphere rich in helium results in a squeaky voice (don't try it!).

Most of the helium existing in the universe today is thought to have been formed in the first few minutes of the big bang with a small amount later produced by stars. It is the second most abundant element in the universe, after hydrogen . .

There is very little helium on earth as nearly all present during and immediately after the earth's formation has long since been lost as it is so light. Just about all the helium remaining on the planet is the result of radioactive decay. While there is some helium in the atmosphere, currently its isolation from that source by liquefaction and separation of air is not normally economic. This is because it is easier, and cheaper, to isolate the gas from certain natural gases. Concentrations of helium in natural gas in the USA are as high as 7% and other good sources include natural gas from some sources in Poland. It is isolable from these gases by liquefaction and separation of from the natural gas. This would not normally be carried out in the laboratory and helium is available commercially in cylinders under pressure.

Most of us feel comfortable with the idea of something floating in water. We see that happen every day. In fact, people themselves float in water, so we have a way of directly experiencing water flotation. The reason why things float in water applies to air as well, so let's start by understanding water flotation.
Let's say that you take a plastic 1-liter soda bottle, empty out the soft drink it contains, put the cap back on it (so you have a sealed bottle full of air), tie a string around it like you would a balloon, and dive down to the bottom of the deep end of a swimming pool with it. Since the bottle is full of air, you can imagine it will have a strong desire to rise to the surface. You can sit on the bottom of the pool with it, holding the string, and it will act just like a helium balloon does in air. If you let go of the string the bottle will quickly rise to the surface of the water.

The reason that this soda bottle "balloon" wants to rise in the water is because water is a fluid and the 1-liter bottle is displacing one liter of that fluid. The bottle and the air in it weigh perhaps an ounce at most (1 litre of air weighs about a gram, and the bottle is very light as well). The litre of water it displaces, however, weights about 1,000 grams (2.2 pounds or so). Because the weight of the bottle and its air is less than the weight of the water it displaces, the bottle floats. This is the law of buoyancy.

Helium Flotation
Helium balloons work by the same law of buoyancy. In this case, the helium balloon that you hold by a string is floating in a "pool" of air (when you stand underwater at the bottom of a swimming pool, you are standing in a "pool of water" maybe 10 feet deep -- when you stand in an open field you are standing at the bottom of a "pool of air" that is many miles deep). The helium balloon displaces an amount of air (just like the empty bottle displaces an amount of water). As long as the helium plus the balloon is lighter than the air it displaces, the balloon will float in the air.
It turns out that helium is a lot lighter than air. The difference is not as great as it is between water and air (a litre of water weighs about 1,000 grams, while a litre of air weighs about 1 gram), but it is significant. Helium weighs 0.1785 grams per litre. Nitrogen weighs 1.2506 grams per litre, and since nitrogen makes up about 80 percent of the air we breathe, 1.25 grams is a good approximation for the weight of a litre of air.

Therefore, if you were to fill a 1-liter soda bottle full of helium, the bottle would weigh about 1 gram less than the same bottle filled with air. That doesn't sound like much -- the bottle itself weighs more than a gram, so it won't float. However, in large volumes, the 1-gram-per-liter difference between air and helium can really add up. This explains why blimps and balloons are generally quite large -- they have to displace a lot of air to float. The following diagram shows the different lifting capacities of different volumes of helium:

A 100-foot-diameter balloon can lift 33,000 pounds! Here is how you can figure out the lifting capacity of the helium in a spherical helium balloon:

Determine the volume of the balloon.
The volume of a sphere is 4/3 * pi * r3, where r is the radius of the balloon. So first determine the radius of the sphere (the radius is half the diameter). Cube the radius (multiply it by itself twice: r*r*r), multiply by 4/3 and then multiply by Pi. If you are measuring your balloon in feet, that gives you the volume of the balloon in cubic feet.
One cubic foot of helium will lift about 28.2 grams, so multiply the volume of the balloon by 28.2.
Divide by 448 -- the number of grams in a pound -- to determine the number of pounds it can lift.
So, for example, a 20-foot balloon has a radius of 10 feet. 10* 10 * 10 * 3.14 * 4/3 = 4,186 cubic feet of volume. 4,186 cubic feet * 28.2 grams/cubic feet = 118,064 grams. 118,064 grams / 448 grams per pound = 263 pounds of lifting force.

Although not used much anymore, hydrogen balloons were once quite popular. Hydrogen weighs just 0.08988 grams per litre. However, it is highly flammable, so the slightest spark can cause a huge explosion.

So why are helium and hydrogen so much lighter than air? It's because the hydrogen and helium atoms are lighter than a nitrogen atom. They have fewer electrons, protons and neutrons than nitrogen atoms do, and that makes them lighter (the approximate atomic weight of hydrogen is 1, helium is 4 and nitrogen is 14). Approximately the same number of atoms of each of these elements fills approximately the same amount of space. Therefore, the gases made of lighter atoms are lighter.

Hot Air
What about hot air balloons? They work by similar principles. If you heat up a gas it expands. In the case of a hot air balloon, when the gas inside the balloon expands the extra gas is pushed out the bottom of the balloon, meaning that there are fewer atoms inside the balloon, meaning that the air in the balloon is lighter than the air outside the balloon.
The amount of lifting power is controlled by how hot the air is. If you heat the air inside the balloon 100 degrees F hotter than the outside air temperature, then the air inside the balloon will be about 25 percent lighter than the air outside the balloon. So a cubic foot of air weighs about 35 grams at 32 degrees F. A cubic foot of hot air at 132 degrees F will weigh 25 percent less, or about 26.5 grams. The difference is 8.5 grams or so. So a hot air balloon has to be much bigger to support the same weight, but it will float because hotter air is lighter than cooler air.

You can get a sense for how much air contracts and expands as its temperature changes by performing the following experiment: Take two Ziplock bags (1-gallon size) and blow them up (you can do this by zipping the bag closed, then unzipping a small hole at one end of the zipper). Blow each bag up like a balloon and seal it while holding pressure on the last breath. You want these bags to be full -- you want the plastic on both inflated bags to be tense.

Now let the bags sit on the counter for a couple of minutes and cool off. You pumped 98.6 degree F air into them, and you want the temperature to drop to room temperature. The bags will probably become a little less tense in the process of cooling (makes sense...) so add one more puff of air to make them tense again.

Now stick one of the bags into your freezer for about three minutes, while leaving the other one on the counter. When you take the bag in the freezer out it will have deflated some. How much? By about 10 percent to 15 percent. It has deflated because cooler air is denser than warmer air. Compare the cold bag to the bag on the counter -- the cold bag will not be tense at all. Then a funny thing will happen as the cold bag warms up -- it will get tense again and return to its original size!

You can clearly see that warmer air takes up more space than cooler air. Therefore, warmer air is lighter than cooler air, and that is what makes a hot air balloon float!

Where Helium Comes From
If you put helium in a balloon and let go of the balloon, the balloon rises until it pops. When it pops, the helium that escapes has no reason to stop -- it just keeps going and leaks out into space.
Therefore, in the atmosphere there is very little helium at any given time. The helium that is there comes from alpha particles emitted by radioactive decay (see How Nuclear Radiation Works for details on alpha decay). In places that have a lot of uranium ore, natural gas tends to contain high concentrations of helium (up to 7 percent). This makes sense, since the decay of uranium emits lots of alpha particles and a natural gas pocket tends to be a sealed container underground. Helium is cryogenically distilled out of natural gas to produce the helium we put in balloons. A litre of air at sea level weighs about 1.25 grams. A litre is 1,000 cubic centimetres, or about 61 cubic inches -- the size of a 1-liter soda bottle. A litre of helium, on the other hand, weighs about 0.18 grams. If you weigh a 1-liter bottle filled with air and then weigh the same bottle filled with helium, it will weigh about 1.07 grams less. If the bottle itself weighed less than a gram, you couldn't weigh it at all -- it would float! You could turn the scale upside down and put it above the floating bottle to check its negative weight! Generally, a balloon has to be several litres in size before the 1-gram-per-litre weight difference of helium vs. air is enough to overcome the weight of the balloon itself and float.

If you could somehow fill a 1-litre bottle with a vacuum, it would float even better. A perfect vacuum weighs zero grams, so a litre of perfect vacuum weighs 0.18 grams less than a litre of helium. The problem, of course, is that building a lightweight container that can hold a vacuum is not nearly as easy as building a fabric envelope that can hold helium. The phrase Nature abhors a vacuum sums it up nicely. If you could figure out a way to do it, however, you would be set -- your vacuum balloon would float!

Note that you would not need to have a perfect vacuum. Any air that you take out of the envelope will lower the weight and cause lift.