In 1913, Henry Moseley confirmed that Mendeleev’s periodic table actually places the elements in sequence by nuclear charge instead of atomic weight and atomic number is the absolute definition of an element. Later, Ernest Rutherford proposed that the integer count of the nuclear charge is identical to the sequence of each element in the periodic table. This inspired the forming of modern periodic table.

Until a Russian professor Dmitri Mendeleev and a German chemist J.Lothar Meyer independently published their periodic tables in 18 respectively. Since then, different ways of classifying known elements emerged, including Law of Triads (by Johann Dobereiner) and Law of Octaves (by John Newlands ).But these findings were not popularly recognised, for not all elements obey these arrangements. His classification was unsuccessful because light and heat are also listed as elements. In modern chemistry, Antoine Lavoiser was the first to classify substances.

The concept of elements was proposed since ancient times.

Period 4 elements