Because not all elements were then known, there were gaps in his periodic table, and Mendeleev successfully used the periodic law to predict properties of some of the missing elements. The first periodic table to become generally accepted was that of the Russian chemist Dmitri Mendeleev in 1869: he formulated the periodic law as a dependence of chemical properties on atomic mass. Other atoms, like nuclides and isotopes, are graphically collected in other tables like the tables of nuclides (often called Segrè charts). The periodic table exclusively lists electrically neutral atoms that have an equal number of positively charged protons and negatively charged electrons and puts isotopes (atoms with the same number of protons but different numbers of neutrons) at the same place. The underlying reason for these trends is electron configurations of atoms. Trends run through the periodic table, with nonmetallic character (keeping their own electrons) increasing from left to right across a period, and from down to up across a group, and metallic character (surrendering electrons to other atoms) increasing in the opposite direction. Elements from the same group of the periodic table show similar chemical characteristics. The rows of the table are called periods, and the columns are called groups. The table is divided into four roughly rectangular areas called blocks. It is a graphic formulation of the periodic law, which states that the properties of the chemical elements exhibit an approximate periodic dependence on their atomic numbers. It is widely used in chemistry, physics, and other sciences, and is generally seen as an icon of chemistry. The periodic table, also known as the periodic table of the ( chemical) elements, is a rows and columns arrangement of the chemical elements.
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