The periodic table of the elements
In the periodic table, all known chemical elements are represented in tabular form. Each box stands for an element, i.e. is a certain kind of atom. Each element has a symbol, e.g. "H" for hydrogen. Furthermore each element has a so-called atomic number. It represents the number of protons of the element. (Protons, together with neutrons, form the atomic nucleus.) The numbering of the periodic table starts in the upper left corner and continues line by line to the lower right corner.