January 22 – Elemental, My Dear Watson!

Today’s Factismal : There are 118 known chemical elements.

One of the great triumphs of science was David Mendeleev’s periodic table of the elements. Though his design was simplified somewhat in 1923 by Horace Deming, who made it easier to see the blocks of similar elements, the basic format remains unchanged since Mendeleev’s first chart.

Mendeleev's original periodic table (Courtesy Sandbh at Wikipedia)

Mendeleev’s original periodic table (Courtesy Sandbh at Wikipedia)

Mendeleev’s work has lasted so long because it does several important things and it does them well. First, it organizes what we know so that it is easier to find out an element’s properties. The first step in any scientific endeavor is discovering what is already known (so that you don’t waste your time rediscovering fire); Mendeleev’s table takes that step for you.

Second, that organization makes it easier to discover natural groupings of the elements which then tells us how one element would react with another based on how similar elements react. Though this may seem obvious now, when the elements were first being discovered in the 1700s and 1800s there were many things that weren’t known. Would oxygen react with hydrogen the same way that it reacted with sodium? Without the periodic table, a chemist had to run the experiment to know the answer to that question; with it, the chemist could confidently say “Yes!” and even predict what the reaction would create.

Third, that organization made it obvious that there were missing elements. In 1870 and 1871 Mendeleev identified five different elements that should have been found but weren’t. Because the elements hadn’t been identified yet, he gave them synthetic names based on the nearest known relative. For example, the missing element that would fill the hole two places down in the table from manganese was called dvi-manganese (“two away from manganese”). Even better, Mendeleev was able to predict the physical properties as well as the chemical ones for the missing element using those of the known elements near them. He could say that eka-aluminum (“one away from aluminum”) would have an atomic mass of 68, a density of 6.0 g/cm3, and have a low melting point. When gallium was discovered, it had an atomic mass of 69.72, a density of 5.9 g/cm3, and a melting point of just 29.78°C. His other four predicted elements (eka-boron, eka-manganese, eka-silicon, and eka-tantalum) were also right on the money (scandium {1879}, technetium {1937}, germanium {1886}, and protactinium{1913}).

An updated version of the table (Image courtesy Brewton-Parker College)

An updated version of the table (Image courtesy Brewton-Parker College)

Chemists and physicists continue to use Mendeleev’s periodic table today as a road map in their search for new elements. If you’d like to follow in their footsteps, then try ChemSpider:
http://www.chemspider.com/

One thought on “January 22 – Elemental, My Dear Watson!

  1. Pingback: April 20 – Big Bang – Little facts about science

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