The periodic "law" of chemistry recognises that many properties of the chemical elements are periodic functions of their atomic number (the number of protons within the element's atomic nucleus). The periodic table is an arrangement of the chemical elements ordered by atomic number in columns (groups) and rows (periods) presented so as to emphasize their periodic properties.
The element polonium is very much in the news at present, perhaps for the first time ever, and for the wrong reasons.
The groups (columns in the periodic table) are numbered 1-18. Some groups enjoy non-systematic names as well. The include Group 1 (alkali metals), Group 2 (alkaline earth metals), Group 15 (pnictogens, or pnicogens), Group 16 (chalcogens), Group 17 (halogens) and Group 18 (noble gases). While not groups in the periodic table, some other groupings of elements are often named as well. These include the lanthanoids (less preferably lanthanides) and actinoids (less preferably actinides).
There are many different ways, sometimes ingenious, of arranging the chemical elements according to which properties are of particular interest but that shown here is a standard form of the periodic table. The relative merits of various other periodic table organisations is still the subject of debate. Particularly useful versions include the following:
Extended periodic table
Janet periodic table
Mendeleev's original periodic table
While the name Dmitri Mendeleev is usually credited with the with the form of periodic table as we know it today, many other excellent researchers made profound contributions to its development, including Antoine Lavoisier, Jöns Jakob Berzelius, Johann Döbereiner, John Newlands, Alexandre-Émile Béguyer de Chancourtois, Lothar Meyer, and others.