Early chemists used the properties of elements to sort them into groups. In 1829, a German chemist, J. W. Dobereiner, published a classification system.
In his system, the known elements were grouped into triads. A triad is a set of three elements with similar properties.
One element in each triad tended to have properties with values that fell midway between those of the other two elements.
Mendeleev's periodic table
In 1869, a Russian chemist and teacher, Dmitri Mendeleev, published a table of the elements. Later that year, a German chemist, Lothar Meyer, published a nearly identical table.
Elements in a periodic table are arranged into groups based on a set of repeating properties. Mendeleev arranged the elements in his periodic table in order of increasing atomic mass.
Modern periodic table
In the modern periodic table, elements are arranged in order of increasing atomic number.
The elements are arranged in order of atomic number, starting with hydrogen, which has atomic number 1. There are seven rows, or periods, in the table.
When elements are arranged in order of increasing atomic number, there is a periodic repetition of their physical and chemical properties. The arrangement of the elements into periods has an important consequence. Elements that have similar chemical and physical properties end up in the same column in the periodic table.
