Dmitri Mendeleev

The arrangement of chemical elements started with Dimitri Mendeleev, a Russian chemist. In 1869 he arranged all the known chemical elements in the order of increasing atomic weights. He found that, for the first 20, each one resembled the eighth element following it in appearance, properties, and activity. Thus lithium, sodium, and potassium are related, as are beryllium, magnesium, and calcium. In the table these series in group Ia and IIa.

Atoms are built in an orderly, progressive manner. Hydrogen, the lightest atom, has a nucleus with one proton (positive electric charge) and one electron to match. adding one positive charge ( and two neutrons) to the nucleus of hydrogen and adding one electron to make an atom of helium. Other additions of protons and electrons in equal numbers (plus neutrons) build up the other kinds of atoms. since neutrons do not affect chemical activity, the number of positive charges identifies the kind of atom. This number is called the atomic number.