Development of Periodic Table:    GO BACK


Should space aliens visit earth, I suggest communications begin with a blank periodic table--anyone capable of space travel would surely recognize the unique shape. So what's the origin of this universally recognized shape or did Mendeleev (father of the periodic table) come down a mountain carrying the famous outline carved in rectangular stone?

The quest for a systematic arrangement of the elements started with the discovery of individual elements (see Discovery of Elements).  By 1860 about 60 elements were known and a method was needed for organization.  In fact many scientists made significant contributions that eventually enabled Mendeleev to construct his table. The periodic table did not end with Mendeleev but continued to take shape for the next 75 years.

The development of the periodic table begins with German chemist Johann Dobereiner (1780-1849) who  grouped elements based on similarities.  Calcium (atomic weight 40), strontium (atomic weight 88), and barium (atomic weight 137) possess similar chemical prepares.  Dobereiner noticed the atomic weight of strontium fell midway between the weights of calcium and barium:
     Ca   Sr   Ba      (40 + 137) 2 = 88
      40     88     137

Was this merely a coincidence or did some pattern to the arrangement of the elements exist? Dobereiner  noticed the same pattern for the alkali metal triad (Li/Na/K) and the halogen triad (Cl/Br/I).
   Li   Na  K         Cl   Br   I
      7     23     39           35    80   127

In 1829 Dobereiner proposed the Law of Triads: Middle element in the triad had atomic weight that was the average of the other two members. Soon other scientists found chemical relationships extended beyond triads. Fluorine was added to Cl/Br/I group; sulfur, oxygen, selenium and tellurium were grouped into a family; nitrogen, phosphorus, arsenic, antimony, and bismuth were classified as another group.

First Periodic Table
It was a 19th century geologist who first recognized periodicity in the physical properties of the elements. Alexandre Beguyer de Chancourtois (1820-1886), professor of geology at the School of Mines in Paris, published in 1862 a list of all the known elements. The list was constructed as a helical graph wrapped around a cylinder--elements with similar properties occupied positions on the same vertical line of cylinder (the list also included some ions and compounds).   Using geological terms and published without the diagram, de Chancourtois ideas were completely ignored until the work of Mendeleev.

Law of Octaves
English chemist John Newlands (1837-1898), having arranged the 62 known elements in order of increasing atomic weights, noted that after interval of eight elements similar physical/chemical properties reappeared.  Newlands was the first to formulate the concept of periodicity in the properties of the chemical elements. In 1863 he wrote a paper proposing the Law of Octaves: Elements exhibit similar behavior to the eighth element following it in the table.


Mendeleev's Periodic Table   
Then in 1869, Russian chemist Dimitri Mendeleev (1834-1907) proposed arranging elements by atomic weights and properties (Lothar Meyer independently reached similar conclusion but published results after Mendeleev).  Mendeleev's periodic table of 1869 contained 17 columns with two partial periods of seven elements each (Li-F & Na-Cl) followed by two nearly complete periods (K-Br & Rb-I).

In 1871 Mendeleev revised the 17-group table with eight columns (the eighth group consisted of transition elements). This table exhibited similarities not only in small units such as the triads, but showed similarities in an entire network of vertical, horizontal, and diagonal relationships. The table contained gaps but Mendeleev predicted the discovery of new elements.  In 1906, Mendeleev came within one vote of receiving the Nobel Prize in chemistry.



Noble Gases

Lord Rayleigh (1842-1919) and William Ramsey (1852-1916) greatly enhanced the periodic table by  discovering the "inert gases."  In 1895 Rayleigh reported the discovery of a new gaseous element named argon. This element was chemically inert and did not fit any of the known periodic groups. Ramsey followed by discovering the remainder of the inert gases and positioning them in the periodic table. So by 1900, the periodic table was taking shape with elements were arranged by atomic weight.  For example, 16g oxygen reacts with 40g calcium, 88g strontium, or 137g barium. If oxygen used as the reference, then Ca/Sr/Ba assigned atomic weights of 40, 88, and 137 respectively.

Rayleigh (physics) and Ramsey (chemistry) were awarded Nobel prizes in 1904.  The first inert gas compound was made in 1962 (xenon tetrafluoride) and numerous compounds have followed (see xenon compounds)--today the group is more appropriately called the noble gases.

Moseley's Periodic Law
Soon after Rutherford's landmark experiment of discovering the proton in 1911, Henry Moseley (1887-1915) subjected known elements to x-rays. He was able to derive the relationship between x-ray frequency and number of protons. When Moseley arranged the elements according to increasing atomic numbers and not atomic masses, some of the inconsistencies associated with Mendeleev's table were eliminated. The modern periodic table is based on Moseley's Periodic Law (atomic numbers). At age 28, Moseley was killed in action during World War I and as a direct result Britain adopted the policy of exempting scientists from fighting in wars.  Shown below is a periodic table from 1930:


Modern Periodic Table
The last major change to the periodic table resulted from Glenn Seaborg's work in the middle of the 20th century. Starting with plutonium in 1940, Seaborg discovered transuranium elements 94 to 102 and reconfigured the periodic table by placing the lanthanide/actinide series at the bottom of the table. In 1951 Seaborg was awarded the Nobel Prize in chemistry and element 106 was later named seaborgium (Sg) in his honor.
Attention: New Additions to Periodic Table 
Physical properties: Generally soft and round in form. Boils at nothing and may freeze any time. Very bitter if not used well.
Chemical properties: Very active and highly unstable. Possesses strong affinity with gold, silver, platinum, and precious stones. Violent when left alone. Turns slightly green when placed next to a better specimen.
Usage: An extremely good catalyst for dispersion of wealth.
Caution: Highly explosive in inexperienced hands!

Physical properties: Solid at room temperature but gets bent out of shape easily. Difficult to find a pure sample. Due to rust, aging samples are unable to conduct electricity as easily as young samples.
Chemical properties: Attempts to bond with WO any chance it can get. Also tends to form strong bonds with itself. Becomes explosive when mixed with Childrium for prolonged period of time.
Usage: Possibly good methane source.
Caution: In the absence of WO, this element rapidly decomposes and begins to smell.



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