Beyond the Book - History of the Periodic Table
Dmitri Mendeleev  (1834-1907)
Born in Siberia, the last of at least 14 children, Dmitri Mendeleev revolutionized our
understanding of the properties of atoms and created a table that probably adorns every
chemistry classroom in the world. After his father went blind and could no longer support
the family, Mendeleev’s mother started a glass factory to help make ends meet. But just as
Mendeleev was finishing high school, his father died and the glass factory burned down.
With most of her other children now out on their own, his mother took her son to St.
Petersburg, working tirelessly and successfully to get him into college.
In the late 1860s, Mendeleev began working on his great achievement: the periodic table of the elements. By
arranging all of the 63 elements then known by their atomic weights, he managed to organize them into groups
possessing similar properties. Where a gap existed in the table, he predicted a new element would one day be
found and deduced its properties. And he was right. Three of those elements were found during his lifetime:
gallium, scandium, and germanium.

Besides his work on general chemical concepts, Mendeleev spent much of his time working to improve
technological advances of Russia. Many of his research findings dealt with agricultural chemistry, oil refining,
and mineral recovery. Dmitri was also one of the founding members of the Russian Chemical Society in 1868,
and helped open the lines of communication between scientists in Europe and the United States.

His greatest accomplishment, however, was the stating of the Periodic Law and the development of the Periodic
Table. From early in his career, he felt that there was some type of order to the elements, and he spent more
than thirteen years of his life collecting data and assembling the concept, initially with the idea of resolving some
of the chaos in the field for his students. Mendeleev was one of the first modern-day scientists in that he did not
rely solely on his own work but rather was in correspondence with scientists around the world in order to receive
data that they had collected. He then used  their data along with his own data to arrange the elements according
to their properties.
Mendeleev also developed what became the Periodic Law, it has 8 parts to it :
  1. The elements, if arranged according to their atomic weights, exhibit a periodicity of properties. Translation - they will form a repeating pattern.
    When this was changed into atomic number we got our current table
  2. Chemically analogous elements have either similar atomic weights (Pt, Ir, Os), or weights which increase by equal increments (K, Rb,
    Cs). Translation: elements with similar properties will have similar weights.
  3. The arrangement according to atomic weight corresponds to the valence of the element and to a certain extent the difference in
    chemical behavior, for example Li, Be, B, C, N, O, F.  Translation: the outer electrons cause elements to bond the way they do - more on this in the
    next chapter.
  4. The elements distributed most widely in nature have small atomic weights, and all such elements are marked by the distinctness of
    their behavior. They are, therefore, the representative elements; and so the lightest element H is rightly chosen as the most
    representative. Translation: the smaller the atom the more of them there seems to be in the universe.
  5. The magnitude of the atomic weight determines the properties of the element. Translation: Change this to atomic number and we have our "the
    number of protons determines the element.
  6. One can predict the discovery of many new elements, for example analogues of Si and Al with atomic weights of 65-75. Translation:
    There are probably more elements we haven't found yet (he was right).
  7. A few atomic weights will probably require correction. Translation: We probably goofed on some of the weights, cuz I said so.
  8. From the above table, some new analogies between elements are revealed. Translation: We can fill in the blanks on the table - ain't I something!
Henry Moseley (1887 - 1915)

Between 1912 and 1914, the physicist H.G.J. Moseley conducted a series of experiments
where he bombarded targets made out of different kinds of metals with cathode rays.
Each metal he studied emitted X-rays of a characteristic frequency, almost like a set of
"fingerprints". He found that the heavier the atomic mass of the element, the shorter was
the wavelength and the more penetrating were the X-rays.

The pattern that emerged when the observed X-rays were organized in order of increasing
frequency suggested to Moseley a regular increase in the positive charge on the nuclei of
the atoms. In passing from one element to the next in the periodic table, the atomic
number (Z) always varied by one unit. Occasional gaps indicated that an element was
missing. In 1914, Moseley published his findings.

Moseley's efforts helped solve riddles which had perplexed Mendeleev and other
scientists for many years. Both the apparent irregularities in the location of such elements
as potassium and argon and the positioning of the rare earth (inner transition) elements in
the periodic table could now be explained on the basis of atomic number.
The modern
periodic law now states that the properties of elements are a periodic function
of their atomic number
. Moseley's methods also permitted a complete survey of the
elements and it showed that, with a few exceptions, the periodic table was complete.

At age 27, Moseley volunteered for military service during World War I and became a
signal officer in the British army. He was killed in action in Gallipoli in 1915.