2.1.1: The Periodic Table

• Johannes Dobereiner noticed that many of the known elements could be grouped in triads of similar properties.
• Now, portions of the periodic table (d block) contain triads of elements.
• By the mid-19th century, the atomic masses of many elements had been determined.
• John Newlands hypothesized that the chemistry of the elements might be related to their masses and arranged the known elements in order of increasing atomic mass.
• Newlands discovered the elements could be classified into octaves (group of seven elements), that correspond to the horizontal rows in the main group elements.
• Newlands's "Law of Octaves" did not work for elements heavier than calcium, and his idea was ridiculed.
• Atomic mass increases with atomic number, and similar properties occur every time a set of ns2np6 subshells is filled.
• The periodic table achieved its modern form through work of Julius Lothar Meyer and Dimitri Mendeleev who focused on the relationships between atomic mass and various physical and chemical properties.
• Meyer aligned the elements according to periodic variations in simple atomic properties, such as atomic volume.
• Atomic volume was calculated by dividing the atomic mass in grams per mole by the density of the element in grams per cubic centimeter.
• Atomic volume is equivalent to molar volume, the molar mass of an element divided by its density.
• Mendeleev did not assume that all the elements had been discovered. Instead, he left blanks in his table in the expectation that more elements would be discovered.
• The groups in Mendeleev's table are determined by how many oxygen or hydrogen atoms are needed to form compounds with each element.
• When the chemical properties of an element suggested that it might have been assigned the wrong place in earlier tables, Mendeleev carefully reexamined its atomic mass.
• Mendeleev's table did not include the noble gases which were discovered by Sir William Ramsey.
• H. G. J. Moseley discovered that the underlying foundation of the order of the elements was the atomic number, not the atomic mass.
• Moseley hypothesized that the placement of each element corresponded to to its atomic number, which is the number of protons in its nucleus.