The "periodic table of elements" is a standardized scientific reference work that lists all of the atomic elements known to modern chemistry. These atomic elements are the fundamental building blocks of all matter in the universe, much the same as the letters of the English alphabet are the fundamental building blocks of all words that are written in the English language. The building blocks of these atomic elements are the three subatomic particles known as protons, neutrons, and electrons; these subatomic particles combine with each other in very predictable ways, following the laws of chemistry and physics to form 109 more or less stable atomic elements.

In theory, there is no limit to the number of possible atomic elements, but there seems to be a practical limitation which is inherent in the physical structure of atoms. Specifically, atomic elements with more subatomic particles are much less common and much less stable than those with a simpler atomic structure. Indeed, efforts to create "new" atomic elements in laboratory experiments typically yield radioactive substances that quickly deteriorate into other, more stable substances. While most naturally occurring atomic elements are relatively stable, they seldom exist as individual atoms. Rather, through a process known as chemical bonding, most atoms share electrons with other atoms to form molecules, a form of matter which is even more stable than atoms.

Keen empirical observation of the ways that atoms combine to form molecules is what led to the creation of the periodic table of the elements by the Russian chemist Dmitri Mendeleev. However, when Mendeleev created his periodic table, he had no knowledge of subatomic structure, so he based his periodic table on the relative weight of the various elements that had been identified in empirical experiments with gases. It wasn't until much later that other scientists determined that the number of protons in a particular element's atomic nucleus was what determined that element's essence and (thus) its ability to form chemical bonds with other elements.