The periodic table of the elements is a tabular method of displaying chemical elements. It is generally thought to have been invented in 1969 by the Russian chemist Dmitri Mendeleev as his way of illustrating recurring, or periodic, trends in the properties of the elements. The layout of the table has been refined and extended over time as new elements have been discovered and new theoretical models have been developed to explain chemical behavior. As a result, it has broad applicability in chemistry, physics, biology, engineering, industry and, of course, radiation protection.