Periodic Table 2

When the periodic table was first taught, there were 108 elements known. This has steadily increased over the years and will no doubt continue to do so. In many cases, only a few atoms have ever been created so we may never know their properties. Only elements 1 to 92 (hydrogen to uranium) occur naturally, the others have been made during nuclear reactions where heavy nuclei are made to interact with each other in particle accelerators.

You have already seen in the introduction to the previous quiz that it was only towards the end of the 18th Century that science had reached a point where the elements could be isolated, identified and cataloged. Early in the 19th Century, Johann Döbereiner started to attempt to classify the known elements on the basis of their properties. He created several groups of three elements (his triads) with similar properties. It was a good start and we see his triads in the modern periodic table. He also noticed that the middle element had an atomic weight that was close to the mean of the other two.

The first person to notice the periodicity of the elements was Alexandre-Emile Béguyer de Chancourtois. He arranged the elements in order of atomic weight in a spiral drawn on a cylinder. He spotted that similar elements lined up vertically. However, his work wasn't recognized until after Mendeleev had published his findings. By the 1860s, John Newlands had arranged the sixty-two known elements into groups of eight with similar peoperties - his 'octaves'. His work was originally ridiculed but it wasn't until the 20th Century that the significance of his work was recognized.

The periodic table as we know it now is based on the work of Dimitri Mendeleev. He also arranged the known elements in groups of 8 horizontally, but with similar properties occuring in the vertical columns. His leap of genius was to leave gaps for elements that he believed had not been discovered and to reverse the order of certain elements where they didn't fit the patterns. There were still many shortcomings of this table, such as where to place hydrogen. His periodic table was widely accepted because he was able to use it to predict the properties of the missing elements. When the noble gasses were discovered, they were added as 'group 0' without disturbing the rest of the table. In the 20th century, knowledge of atomic structure enabled scientists to make more and more sense of how the elements should be arranged, leading to the periodic table that you use in your lessons today.