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Periodic table a guide to the universe

Almost every science classroom I have ever been in contains a periodic table. It is one of the most recognizable of all scientific images.
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Almost every science classroom I have ever been in contains a periodic table. It is one of the most recognizable of all scientific images. In part, this is due to its peculiar shape which is a consequence of chemistry ultimately being about electrons in valence shells.

The construction of the table is also a function of atomic number or the number of protons found within the nucleus of each atom of each element. Every atom of carbon has six protons while every atom of technetium has 43. The number of protons distinguishes the elements from one another.

But in the early 1800s, scientists were struggling to understand the nature and relationships amongst all of the elements which had been discovered. They recognized some patterns and similarities in the chemical properties but did not have an overall picture. By the mid-1850s, the situation was becoming sufficiently perplexing that a conference was called to discuss the state of understanding.

At the 1860 Karlsruhe Conference, Stanislao Cannizzaro proposed using atomic weight (or relative atomic masses) as the guiding principle. Nine years later, and exactly 150 years ago, Dimitri Mendeleev used the known atomic weights and the chemical properties of the elements to publish the first version of the modern periodic table.

It wasn't presented in a form which most of us would recognize. Among other issues, it had blank spots left for undiscovered elements. But Mendeleev was sufficiently confident in the process he had used to assign the elements to his table that he was able to make predictions about the physical and chemical properties of the undiscovered species. His work resulted in the discovery of numerous elements.

If you have ever spent any time staring at the table, you might have noticed some strange anomalies. The most obvious is the atomic weights listed don't match the atomic number and they are not whole numbers. Hydrogen is listed as 1.00797 and not simply 1.0. Indeed, in three cases (Ar-K, Co-Ni, Te-I), a heavier element comes first on the table. For example, Tellurium has an atomic weight of 127.6 grams/mole while Iodine is only 126.9 grams/mole. Nevertheless, the chemistry of Iodine is consistent with the other halogens while Tellurium fits into the chalcogenides.

As confounding as these results were, in the late 1800s, the precision of the data about the atomic weights of the elements wasn't sufficient to invalidate the results provided by the table. These anomalies were left "unexplained" although not from a lack of trying on the part of the scientific community. The periodic table, though, withstood the questions. After all, Mendeleev's unknown elements had been found with chemical and physical properties matching his predictions.

In the early 1900s, Ernst Rutherford, Hans Geiger, and Ernest Marsden conducted experiments revealing the structure of the atom - a positively charged nucleus surrounded by a cloud of electrons. In 1913, using core electron x-ray spectroscopy, Henry Moseley determined the atomic number was the number of protons in the nucleus of an atom. The underlying structure of the periodic table was not the atomic weight of the elements but the atomic number. An explanation for the anomalies of the atomic weight inversions had been found. Tellurium precedes Iodine because it has 52 protons in its nucleus compared to iodine's 53.

In the meantime, Niels Bohr was a post-doctoral fellow working with Rutherford tackling the questions raised by the solar system model of the atom. Specifically, such a model should lead to the electron losing energy in the form of x-rays. The electrons should quickly collapse into the nucleus. Bohr's work resulted in the development of quantum mechanics and the behaviour of electrons within atoms. It is Bohr's model which gave the periodic table its modern shape.

This is not to say everything was solved. With the exception of hydrogen, all of the elements have multiple positively charged particles in their nucleus and these should repel each other blowing the core of the atom apart. Further, the atomic mass didn't make sense because if each atom was simply a multiple of hydrogen atoms, then the masses should also be a multiple of hydrogen's.

In 1932, James Chadwick found the answer when he discovered the neutron. Atoms contain three types of sub-atomic particle - the positively charged proton, the negatively charged electron and the neutral neutron. The neutron provided a mechanism for holding nuclei together. It also led to the realization that elements have isotopes - multiple forms where the number of protons remains constant but the number of neutrons varies. For example, every atom of tin has 50 protons in its nucleus but with somewhere between 49 and 87 neutrons - although only ten between 112 and 124 are stable.

The periodic table isn't just a poster on the wall but the gateway to understanding the world around us.