Yesterday, The Nine-Year-Old came home from science class bearing wild tales of hydrogen atoms with extra neutrons in them. I had forgotten that this was possible. It made me wonder if those extra neutrons were what made up the heavy water that I vaguely remembered was an important component of certain types of nuclear reactors. So I asked Dr. Google, Ph.D.
The short answer is yes. Heavy water is water in which one or more of the regular hydrogen atoms in the water molecule has been replaced with a heavier hydrogen isotope called deuterium.
In case you, like me, need a refresher on what all that means, please read on.
(Disclaimer: The following is excessively simplified and no substitute for a real conversation with an actual scientist.)
What are molecules, again?
Most of the world around us is made up of molecules that are in turn a mix of simpler substances. Water molecules, for example, are a blend of hydrogen and oxygen. Table salt is made up of sodium and chloride. In its most basic form, steel is a mix of iron and carbon. You get the idea.
Molecules like the ones in water, salt, and steel can be broken down into the different substances that make them up. You can use electrolysis to split water into hydrogen and oxygen, for example.
What’s an element?
Unlike molecules, elements cannot be broken down into simpler substances. If you divide a clump of oxygen, you will simply end up with two smaller clumps of oxygen.
So what are atoms?
Elements are made up of atoms, tiny particles that all exhibit the same properties as the element they make up.
For a long time, scientists thought that atoms were the smallest particles in the world. But in 1897, while experimenting with electricity passing through glass tubes, J. J. Thompson proposed that the cathode rays that streamed through his empty glass tubes were actually made up of even smaller particles, which he called corpuscles. This name, surprisingly, didn’t catch on. Later experiments convinced Thompson that electron was a more appropriate name for these very small, negatively charged particles inside atoms.
In 1911, Ernest Rutherford’s experiments on radioactivity convinced him that the atom’s collection of negatively charged electrons must be balanced in some way by a mass of positively charged particles, or protons, in the atom’s core. Our buddy hydrogen, for example, has one proton in its nucleus, and one electron, which orbits the nucleus.
Over time, however, it became clear that the proton wasn’t the only particle in an atom’s nucleus. The atoms Rutherford and his team studied weighed more than the scientists would have expected them to, based on the number of electrons and protons that made them up. Something else must be contributing to the atom’s mass.
Thanks to the work of Thompson, Rutherford, and Chadwick, the definition of an atom had to be refined. No longer the smallest particles in the world, atoms are instead the smallest particles of an element that still retain all of that element’s properties.
For the most part, an atom’s nucleus contains a consistent number of protons and neutrons. But not always. Isotopes are versions of an atom that have a different number of neutrons than you would typically expect (either more or less). Which brings us at long last to today’s question.
What is heavy water?
Our buddy hydrogen turns out to be far more interesting than he first appears. While the vast majority of hydrogen atoms indeed have only one proton and one electron in them, there are some varieties of hydrogen that have extra neutrons.
Hydrogen has three of these variations, known as isotopes. The most common isotope of hydrogen, protium, has one proton, one electron, and no neutrons. Deuterium has one proton, one electron, and one neutron. The third isotope, tritium, has one proton, one electron, and two neutrons.
Deuterium is stable, naturally occurring (although rare), and non-radioactive. Tritium, on the other hand, is unstable and, therefore, radioactive. It’s also quite rare, typically occurring as the by-product of cosmic rays hitting our atmosphere or as the result of irradiating lithium in a nuclear reactor.
Their extra neutrons make both deuterium and tritium heavier than the standard hydrogen atom (protium).
Which brings us to heavy water. Regular water molecules are made up of two regular hydrogen (protium) atoms combined with an oxygen atom. To create heavy water, you simply replace one or more of those protium atoms with the heavier deuterium.
Although I associate heavy water with nuclear reactors, heavy water occurs naturally in the Earth’s oceans as well. Roughly one in 6400 of the hydrogen atoms in the Earth’s oceans are actually deuterium atoms. Which means that while the vast majority of the Earth’s ocean water is regular water, some insignificant portion of it is actually heavy water. According to About Chemistry, for every twenty million regular water molecules you’ll find in the Earth’s ocean, there will be one heavy water molecule.
So can I drink it?
Heavy water is drinkable in the small quantities in which it occurs in nature, so don’t spend too much time worrying about whether or not it’s in your drinking water. But don’t replace your regular water with concentrated heavy water either.
Drinking heavy water in small quantities might make you dizzy by changing the weight of the fluid in your inner ear. But drinking heavy water in large quantities over a long period of time is dangerous in the same way that being exposed to radiation is dangerous.* Due to its extra neutrons, heavy water behaves differently in biochemical reactions than regular water will. When consumed in large quantities, heavy water interferes with your cells’ ability to repair their DNA and replicate. No one wants that.
*Update: As Susan points out in the comments below, this bit is rather sloppy wording. Drinking large volumes of heavy water and being exposed to radiation are both dangerous, but they aren’t dangerous in *exactly* the same way.