|
The periodic table |
|
n 1799 Joseph-Louis Proust established that the compound copper carbonate always has the same proportions of copper to carbon to oxygen by mass, no matter how the compound is prepared. He proceeded to carry out similar analyses of other compounds over the next ten years, showing that in every case the proportions of the elements by mass stayed constant for a given compound. John Dalton was the first to explain why Proust's law of definite proportions held true. Dalton postulated in 1800 that the chemical elements are made from atoms that differ largely by mass. If the mass of an atom of hydrogen, the lightest element, is taken as 1, then the other elements have atoms whose masses are multiples of hydrogen. Dalton tried to calculate these masses, but got in trouble because he was not sure of the exact atomic composition of compounds, such as water. In 1828, however, Jöns Jakob Berzelius published a list of atomic weights (weighted averages of atomic masses found in natural sources of an element) that was quite accurate. No one paid much attention to this idea at the time. In 1860 Friedrich Kekulé reached the conclusion that chemistry was in chaos because different formulas were being used for the same compounds. He organized the first international scientific congress, the First International Chemical Congress, to straighten things out. The highlight of the meeting was an address by Stanislao Cannizzaro that stressed the importance of atomic weights. A few scientists began to list the known elements in the order of their atomic weights. When they did, they found that roughly every eighth element was similar. The first to publish this information was Alexandre-Emile Beguyer de Chancourtois in 1862, but the article failed to reproduce his diagram. Without a diagram, the periodicity of the elements was far from clear. The next year John Newlands announced his version, which he called the law of octaves. People were unimpressed and claimed that just as much periodicity could be seen if the elements were listed in alphabetical order.
|