HISTORY OF THE PERIODIC TABLE

 Elements like gold, silver, tin, copper, lead, and mercury have been known for thousands of years.  The first scientific discovery of an element was in 1649 when phosphorus was discovered.  Over the next 200 years, chemists discovered more elements along with properties of those elements.

 By 1869, a total of 63 elements had been discovered  Scientists began to recognize patterns in the properties of the elements and started trying to classify the elements in various ways.

Law of Triads  Johan Dobereiner proposed that nature contained triads of elements in which the middle element had properties that were an average of the other two elements.

 Other scientists found similar types of relationships extending beyond triads. But, research was hampered by a lack of accurate values for atomic masses.  In 1860, an Italian chemist named Stanislao Cannizzaro presented a method for accurately measuring relative atomic masses. This method enabled scientists to agree on mass values and find relationships among the elements.

 A French geologist, de Chancourtois, was one of the first scientists to propose that the properties of the elements were related to their atomic masses, and that these properties tended to reoccur in a regular pattern.

Periodicity  The definition of periodicity is the tendency to recur at regular intervals.  Many properties of the elements follow a periodic pattern and that is the basis of the arrangement of the periodic table.

Law of Octaves  John Newlands classified 56 elements into groups based on similar physical properties, and saw that many pairs of elements with similar properties had atomic masses that differed by a multiple of eight. He called this the law of octaves because musical notes also come in groups of eight.

 Two scientists produced similar periodic tables during the 1860s.  Lothar Meyer published an abbreviated version in a textbook – but his list only included about half the known elements.  Dmitri Mendeleev published his version of a periodic table in his textbook. Because his version was more complete, was available to the scientific community earlier than Meyer’s, and his predictions about un- discovered elements proved true, Mendeleev is considered the “Father of the periodic table.”

Mendeleev  Mendeleev created cards for each of the 63 known elements.  Each card contained the element’s symbol, atomic mass, and its characteristic chemical and physical properties.  He arranged the cards in order of increasing atomic mass, grouping elements with similar properties together.

 Mendeleev’s arrangement left several blank spaces. He predicted that new elements would be discovered that would fit in those blanks. His prediction proved remarkably accurate.  But, Mendeleev had to change the order of several elements.  For example, he had to put iodine with a mass of 127 after tellurium with a mass of 128.

 In the early 1900’s, Henry Moseley was working with Rutherford – observing the line emission spectrums of various elements.  Moseley noted that the elements fit into patterns better when they were put in order of increasing atomic number instead of atomic mass.

 During the 1890’s, Lord Rayleigh discovered a new gaseous element called argon which did not fit into any of the known periodic groups.  In 1898, William Ramsey suggested that argon be put into a new group along with helium – establishing the noble gases. Ramsey went on to discover neon as well.

 The next change in the periodic table occurred in the early 1900s with the addition of the lanthanides. Many scientists contributed to the discovery of these elements and their properties.

 Glenn Seaborg discovered plutonium in 1940, and went on the discover all of the transuranic elements from 94 to 102. He placed the actinides below the lanthanide series.  Element 106 was named Seaborgium in his honor.

 The periodic table is arranged by atomic number, which places elements with similar properties in groups.  Periodic means a repeating pattern. So the properties of the elements tend to repeat in each row (period).  Groups of elements tend to have similar physical and chemical properties.