Enough about religion and all that. This is supposed to be a blog about wonder. For the next twelve nights I intend to write about possibly the most wondrous of all the elements, helium. As Joseph Campbell said, “I don’t believe in being interested in a subject just because it’s said to be important. I believe in being caught by it.” Maybe by the twelfth night, helium will have caught you.
Let’s start with an obvious one. Why do helium balloons float? This simple question will take us to the brink of one of the great discoveries of science: all the world is made of atoms.
Imagine yourself holding a string tied to a helium balloon. You feel that string tugging on your fingers, begging to be released. The helium balloon wants to escape, to fly away. But this is an illusion. Helium, like all matter, is affected by gravity. The gravitational field of the Earth pulls on helium just as surely as it pulls on everything else. So whence the illusion of lift?
Gravity pulls on air, too. But air is heavier than helium, and so the air around and above the helium balloon is pulled more strongly by the Earth than is the helium. You’ve experienced the same thing if you’ve ever held a plastic ball under the surface of the water in a pool or a bathtub. The water pushes below the ball, and so the ball seems to want to rise up.
But this leads to a lovely problem. Why should helium be lighter than air? To get to that answer, we have to take a roundabout path (those are the best kind, aren’t they?) through two more elements, hydrogen and oxygen.
Suppose you take some water, add some sodium sulfate to it to make it a good conductor, and run an electric current through. It doesn’t take much power, a six-volt lantern battery will do nicely. If you do this, from the positive end of the battery you’ll get bubbles, and from the negative end you’ll get more bubbles, twice as many, in fact. Simple tests will show you that the lesser bubbles are oxygen gas, while the double bubbles (ha!) are hydrogen. And a really fun test, recombining the hydrogen and oxygen (kaboom!) will show that every bit of each element will be used up, resulting in water once again.
This suggested to scientists that 1) everything was made of tiny particles, eventually called atoms and 2) the atoms combined in simple ratios to form things like water. In fact, as of course you know, the formula for water is H2O, two H atoms for every one O atom.
But notice that the volumes of the gases work out just the way you might hope. The volume of hydrogen gas is just twice the volume of oxygen gas. This idea eventually became Avogadro’s hypothesis. Simply, it says that as long as everything else is the same, equal volumes of two different gases contain the same number of atoms (or molecules, if the gas is, like both hydrogen and oxygen, molecular).
OK, fine. That means the number of atoms inside our helium balloon are the same as the number of molecules of air in an air balloon. And yet we know from experience that helium balloons float, while air balloons do not. What gives?
The difference must be with the atoms themselves. The world is made of atoms, and those atoms have different properties. The reason helium floats, makes your voice squeak, and stubbornly resists reacting with even the most reactive of chemicals, is that helium atoms have certain specific properties. Helium balloons float because helium atoms are very, very light. Soon we’ll find out why, and that journey will take us not just to the atoms themselves, but actually inside. Stay tuned . . .