The Periodic Table, originated by the scientists Dmitri Mendeleyev (see picture) and Lothar Meyer, is a table in which the chemical elements are arranged in order of increasing proton number (also known as atomic number). The proton number is the number of protons in the nucleus of an atom. Each element has a unique proton number. The arrangement of the table is such that elements with similar electronic configurations are grouped together, and chemical relationships between the elements are emphasized. The principle underlying the table is the Periodic Law, which states that the properties of the elements are a periodic function of the proton numbers. This means that elements with similar properties appear together in the same group.
The horizontal rows of the table are called periods. Each - except for the first - begins with an alkali metal, and ends with a noble gas. The seven periods are these: from hydrogen to helium; lithium to neon; sodium to argon; potassium to krypton; rubidium to xenon; caesium to radon; and from francium onwards. The elements necessary to complete this last period have not yet been discovered.
From left to right across most periods, there is a movement from metallic to non-metallic elements. A staggered line to the right of aluminium, germanium, antimony and polonium, divides the metals from the non-metals, although several elements on either side of this line are metalloids, with properties both typically metallic and non-metallic.
The vertical rows of the table are called groups. There are eighteen of these. The co-members of each group display similarities in their electronic configurations, and tend to have similar properties.
