Periodic table




Antoine Lavoisier recognized and named hydrogen. Lavoisier helped construct the metric system, wrote the first extensive list of elements, and helped to reform chemical nomenclature.

Dobereiner's Periodic Table


A German scientist called Johann Dobereiner put forward his law of triads. Each of Dobereiner's triads was a group of three elements. The appearance and reactions of the elements in a triad were similar to each other.

A periodic table


Julius Lothar Meyer had produced a table of just 28 elements which he listed by their valence. The 28 elements were almost entirely main group elements. He incorporated transition metals in another table in 1868 which listed the elements in increasing weight order with elements with the same valence in a given column. This was earlier than Mendeleev's table but unfortunately Meyer's was not published until 1870.

Law of octaves


John Newlands put forward his law of octaves. He arranged all the elements known at the time into a table in order of relative atomic mass. When he did this, he found that each element was similar to the element eight places further on.

Periodic table was published


Russian chemist called Dmitri Mendeleev published a periodic table. Mendeleev also arranged the elements known at the time in order of relative atomic mass, but he did some other things that made his table much more successful.

Discovery of argon, helium and other noble gases


Ramsay’s curiosity was piqued by Lord Rayleigh’s observation that the density of nitrogen extracted from the air was always greater than nitrogen released from various chemical compounds. Ramsay then set about looking for an unknown gas in air of greater density, which—when he found it—he named argon. While investigating for the presence of argon in a uranium-bearing mineral, he instead discovered helium. This second discovery led him to suggest the existence of a new group of elements in the periodic table. He and his coworkers quickly isolated neon, krypton, and xenon from the earth’s atmosphere.

Cathode rays


J. J. Thomson set out to prove that the cathode rays produced from the cathode were actually a stream of negatively charged particles called electrons. From Maxwell's theory, he knew that charged particles could be deflected in a magnetic field.

Discovery of electron orbits


Niels Bohr was the first to discover that electrons travel in separate orbits around the nucleus and that the number of electrons in the outer orbit determines the properties of an element.

Charge of the nucleus


A. van den Broek suggested that it was a mistake to compare the charge on the nucleus with the atomic weight of the element. He suggested that the charge should be compared with the atomic number, which specifies the position of the element in the periodic table. Within experimental error, the estimate of the charge on the nucleus obtained from a-particle scattering experiments is equal to the atomic number of the element.

Organising principle


Using atomic number instead of atomic mass as the organising principle was first proposed by the British chemist Henry Moseley in, and it solved anomalies like this one. Iodine has a higher atomic number than tellurium