Dmitri Mendeleev and the periodic table
In the 19th century, there was a Russian chemist named Dmitri Mendeleev. He was a professor at the University of St. Petersburg when he discovered patterns in the properties of certain elements. He developed a chart called the periodic table of elements. (The word 'periodic' refers to something that occurs at regular intervals.)
Mendeleev was not the first to notice these patterns. In the early 19th century, chemists like Johann Dobereiner noticed that certain groups of three elements had similar properties, and that these elements had atomic weights that were evenly spaced. For example, calcium, strontium and barium all have similar properties, and the atomic weight of strontium is around the average of the weights of calcium and barium. He called these similar groups triads.
Mendeleev, however, realised that similarities between elements went beyond groups of three. He developed a chart that showed groups of elements that were similar. This chart was called the periodic table. The rows were arranged in increasing atomic weight, and the columns showed elements that had similar properties. These columns were referred to as groups. This showed a continuous pattern throughout the known elements of the time. See Image 1.
In the 19th century, about 60 different elements had been discovered. Today, we know of more than 110. As a result, there were some gaps in Mendeleev's periodic table. Mendeleev explained this by saying that these spaces could be filled by elements that had not yet been discovered. His colleagues were a little sceptical at first, but during Mendeleev's lifetime, a few of his predicted elements were discovered: gallium, scandium and germanium. Due to those discoveries, Mendeleev's periodic table became widely accepted throughout the scientific community.
Mendeleev's table had some problems with it, however. There were some elements that seemed to be out of sequence - in other words, they had atomic weights that were unexpected because of their properties (Refer to Topic 1, Chapter 6).
Another problem arose when, in 1895, a chemist named Lord Raleigh found an element called argon that did not fit in with any of the groups. Argon, a gas at room temperature, is completely inert, which means it doesn't react with any other elements. Another scientist named William Ramsey noticed it had similar properties to those of helium, and suggested they both be placed in their own column on the far right of the periodic table. This group was called the noble gases. Today we have discovered more noble gases, including neon, krypton, xenon and radon. There are more than 110 elements on the periodic table, and there are spaces in today's periodic table that may one day be filled with new elements.