In 1789, building upon the work of precursors and contemporaries alike, the French chemist Antoine Laurent Lavoisier first defined an element as a fundamental substance that could not be broken down by any chemical means then known. In the same Treatise on Chemical Elements, he compiled a list of 33 elements (a number of which were not actually elements) and devised a naming system for the discovery of new elements.
Lavoisier's definition and list of elements helped spur an attempt by chemists to systematize and understand the elements. In 1803, the English chemist John Dalton used the general scientific recognition that elements combined with each other according to different ratios by weight to create an atomic theory that claimed all elements were built out of variable numbers of hydrogen atoms. As a part of this theory, Dalton created a scale of atomic weight based on the hydrogen atom (the weight of hydrogen was set equal to 1). In 1869, the Russian chemist Dmitry Mendeleyev organized the elements in a table according to their atomic weights (the German chemist Julius Lothar Meyer independently struck upon the same organization in 1870).
In the sixty-seven years from Dalton's formulation of atomic weight to Mendeleyev's periodic table many scientists had tried to create a working organizational structure for the elements. Mendeleyev succeeded where others failed because he realized that there existed a number of as yet unknown elements with atomic weights between the weights of already known elements. By leaving vacancies for those elements he believed were undiscovered, he hit upon an organizational scheme that seemed to vertically group elements with similar properties. Among elements with low atomic weights, he found that similar chemical characteristics recurred every seven elements. Among heavier elements, he found that characteristics resurfaced every seventeen elements. This phenomenon in which physical and chemical characteristics of elements are periodic functions of their atomic weight is called the periodic law (and gives the periodic table its name). In 1879, Mendeleyev's periodic table received a powerful boost in general acceptance when it predicted the existence of the elements gallium, germanium, and scandium.
Through time, Mendeleyev's periodic table has undergone some small changes. Many, many new elements have been added. The discovery of the inert gases raised the number of elements between similar elements to eight for the lighter elements and eighteen for the darker elements. In a few instances, scientists have discovered that organization along atomic weights does not coincide with vertical similarities. In such instances, as in the case of tellurium (Te) and iodine (I), similarity wins out over atomic weight in determining organization.
The next section will describe how to read the periodic table.