Imagine going to the library and finding all the books in big messy piles like the one above. It could take a very long time to find the book you wanted. You might give up without even trying. Of course, in most libraries, books are arranged in an orderly way, like the books shown below. For example, novels, like those pictured here, are arranged in alphabetical order by author’s last name. Not only can you quickly find the book you want, you can also scan the books nearby to find others by the same author. It’s clear that grouping books in an organized way is very useful.

The same is true of chemical elements. For many years, scientists looked for a good way to organize them. This became increasingly important as more and more elements were discovered. In this chapter, you’ll read how elements were first organized and how they are organized today. You’ll see why an orderly arrangement of elements, like the books in a library, is also very useful.