History

For the periodic table to be constructed the individual elements that would constitute this table had to be discovered. Although elements such as gold, silver, tin, copper, lead and mercury had been known for hundreds of years, the first scientific discovery of an element didnt occur until 1649 when Hennig Brand discovered phosphorous. As the next two centuries passed, new elements were being discovered all the time, with the properties and natures starting to be understood by scientists. By 1869, a total of 63 elements had been discovered. As the number of known elements grew, scientists began to recognize patterns in properties and began to develop classification schemes. The basis for a Periodic table of elements had been created. Two laws were postulated that helped construct the Periodic table that we see today.

Law of Triads

In 1817 Johann Dobereiner noticed that the atomic weight of strontium fell midway between the weights of calcium and barium, and that these three elemnts had similar properties. His work in this area continued with the discovery of the halogen triad composed of chlorine, bromine, and iodine and the alkali metal triad of lithium, sodium and potassium. From these observations he proposed that some elements existed as triads of elements, in which the middle element had properties that were an average of the other two members when ordered by the atomic weight. This then became known as The Law of Triads.

The Law of Triads became an increasing area of interest for a number of scientists such as. Between 1829 and 1858 a number of scientists such as Jean Baptiste Dumas, Leopold Gmelin and Ernst Lenssen, Their research led them to the conclusion that these types of chemical relationships extended beyond the triad. For example fluorine was added to the halogen group; oxygen, sulfur,selenium and tellurium were grouped together and nitrogen, phosphorus, arsenic, antimony, and bismuth became another such group. This sort of research was hindered by the limit of accurate values for the various elements.

Law of Octaves

John Newlands, an English chemist, wrote a paper in 1863 which classified the 56 established elements into 11 groups based on similar physical properties, noting that many pairs of similar elements existed which differed by some multiple of eight in atomic weight. This led to Newlands constructing his version of the periodic table and from this the Law of Octaves (by analogy with the seven intervals of the musical scale) was postulated. This law stated that any given element will exhibit analogous behavior to the eighth element following it in the table.