Hydrogen, the lightest atom, has a nucleus with one proton (positive electric charge) and one electron to match. But discoveries of other elements filled the gaps, and the use of atomic numbers removed the contradictions.Ītoms are built in an orderly, progressive manner ( see atomic particles). Mendeleev’s table had many gaps, and the order by atomic weight introduced several contradictions in the sequence of chemical properties for some elements-cobalt and nickel, for example. In the table these series appear in Groups Ia and IIa. Thus lithium, sodium, and potassium are related, as are beryllium, magnesium, and calcium. He found that, for the first 20, each one resembled the eighth element following it in appearance, properties, and activity. In 1869 he arranged all the known chemical elements in the order of increasing atomic weights. The arrangement of chemical elements started with Dmitri Mendeleev, a Russian chemist.