A list of chemical elements arranged along horizontal rows in increasing atomic number. It is organised such that the vertical columns consist of elements with remarkably similar properties. The first column, known as the alkali metals (albeit with hydrogen, a nonmetal on top), contains elements with just one outer (valence) electron. The last column has completely filled valence orbitals leading to chemically inert elements called the noble gases. The position of elements in the periodic table provides a powerful method of classifying not only the physical properties of elements but also their expected properties in molecules and solids.