Antoine-Laurent de Lavoisier
| Published a textbook that contained a list of elements, or substances that could not be broken down further, which included oxygen,nitrogen, hydrogen, phosphorus, mercury, zinc, and sulfur. It also forms the basis for the modern list of elements.
| Johann Wolfgang Döbereiner
| Found that some elements formed groups of three with related properties. He termed these groups "triads". In all of the triads, the atomic mass of the second element was almost exactly the average of the atomic weights of the first and third elements.
| Alexandre-Emile Béguyer de Chancourtois
| The first person to notice the periodicity of the elements — similar elements seem to occur at regular intervals when they are ordered by their atomic weights. He devised an early form of periodic table, which he called the telluric helix.
| John Newlands
| Classified the 56 elements that had been discovered at the time into eleven groups which were based on similar physical properties. He also noted that many pairs of similar elements existed which differed by some multiple of eight in mass number, and was the first person to assign them an atomic number.
| Dmitri Mendeleev
| The first scientist to make a periodic table much like the one we use today. arranged the elements in a table ordered by atomic mass, corresponding to relative molar mass as defined today.
| Lothar Meyer
| Also worked on a table that only included 28 elements. He classified elements not by atomic weight, but by valence alone.
| Henry Moseley
| Found a relationship between an element's X-ray wavelength and its atomic number (Z), and therefore resequenced the table by nuclear charge rather than atomic weight. He showed that there were gaps in his table at atomic numbers 43 and 61 which are now known to be Technetium and Promethium, respectively, both radioactive and not naturally occurring.
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