The periodic table classifies chemical elements by their similarities and properties. Each square in the periodic table shows an element's atomic number, its symbol, its name and its atomic weight, which is the number of protons and neutrons the element has.
Every chemical element has a nucleus comprised of protons and neutrons. Electrons orbit around the nucleus. The elements in each horizontal row are all members of a family. Elements in the same family share similar properties. They're arranged in order of increasing atomic number. All elements in the vertical column are members of the same period. "Period" refers to periodic patterns in chemical and physical properties observed when elements are arranged according to increasing atomic number, according to HowStuffWorks.
The modern periodic table was formulated by Russian chemist Dmitry Mendeelev. Earlier attempts to categorize the elements failed because scientists were ignorant of atomic structure. Knowledge of atomic structure made it possible to arrange the elements in periods, which describe the number of electrons per shell around the nucleus. All of chemistry is defined by interactions with electrons. Since elements in the same vertical period have similar electron shells, it is possible to predict their chemical reactivity before it happens.