A few elements, many compounds

There are only about a hundred elements, and yet chemists have described millions of compounds. A small stock of building blocks is producing an enormous range of substances.

That is less strange than it sounds. An alphabet of twenty-six letters writes every book in a language, because what matters is not how many letters there are but how many ways they can be put together. Elements behave the same way: sodium and chlorine are two elements, but combined they are table salt, a substance neither of them resembles.

The elements that will not join in

The pattern has exceptions, and they are the interesting part. A few elements form almost no compounds under ordinary conditions. Helium, neon and argon are found in nature as single, separate atoms drifting about as gases — never paired with anything, not even with each other.

These unreactive elements are called the noble gases. Sodium, by contrast, reacts so readily that it has to be stored under oil, and chlorine attacks most metals it meets.