Unit 8 Historical development of the periodic table (2.5) Current state of the periodic table (2.5)

Historical Status (2.5) By the mid-1800’s about 60 elements were known Due to the work done with the laws of definite proportions and multiple proportions, among others, the relative masses of atoms of the elements were fairly well-known Because science has a tendency toward organization and classification, it was necessary to find a method to catalogue these elements.

Developments Prior to the Current Periodic Table Dobereiner’s Triads (1816) Johann Dobereiner noticed similarities in trios of some elements. For example, lithium, sodium, and potassium have many similar properties Table from http://www.corrosion-doctors.org

Developments Prior to the Current Periodic Table De Chancourtois’s “Telluric Helix” (1862) Beguyer de Chancourtois observed that if the elements were arranged by relative atomic masses and wound in a spiral fashion, elements with similar properties would be aligned vertically. Images from http://www.corrosion-doctors.org

Developments Prior to the Current Periodic Table Newland’s Law of Octaves (1863) John Newlands noticed that when the elements were listed in order of atomic mass every eight element had similar properties. This only worked well through calcium and placed iron in the same group as oxygen and sulfur even though their properties are very different. Table from http://www.corrosion-doctors.org

Developments Prior to the Current Periodic Table Lothar Meyer (1868) Lothar Meyer came up with a periodic chart very similar to the one used today. His work was at the same time as that of the ultimate developer of the periodic table, Dmitri Mendeleev, but Mendeleev manage to publish his first and is typically given the most credit. Lothar Meyer’s Periodic Table - 1870

The Current Periodic Table (2.5) Mendeleev (1869) The current periodic table is widely attributed to Dmitri Mendeleev. Meyer’s table was very similar but Mendeleev published his a few months before Meyer and Mendeleev’s had recognized predictive value. Two points regarding Mendeleev’s: The elements were arranged mostly by atomic masses, but he switched a couple around to keep elements with similar properties together. Mendeleev left a couple of openings with the belief that those elements had not yet been discovered. He predicted the existence of “eka-silicon” and its properties – eventually this element was discovered and named germanium. Property Predicted for eka-silicon by Mendeleev (1871) Observed by Winkler for Germanium (1876) Atomic mass 72 72.6 Density (g/cc) 5.5 5.47 Color Dirty gray Grayish white Density of oxide (g/cc) 4.7 4.703 Boiling point of chloride Below 100 °C 86 °C Density of chloride (g/cc) 1.9 1.887

The Current Periodic Table (2.5) Mendeleev (1869) Below is Mendeleev’s first periodic table Compare this to today’s periodic table (front flap of your book or next slide) and you will see a striking similarity.

Historical Discovery of Elements Table from IUPAC.org The number of known elements has grown considerably since Mendeleev’s time. The chart above gives a sense of the time period of elemental discovery – in Mendeleev’s time he basically had the gray and green boxes above to work with.

The Current Periodic Table (2.5) Much of our study of chemistry will revolve around the elements, their combinations, and properties based on position in the periodic table.