Introduction
Periodic Table is an organised array of the elements in increasing atomic number, which means increasing number of protons in the nucleus of each elements' atom. This type of arrangement causes a repetition of properties within the table which allows further classification of the elements into groups of certain properties. However, the periodic table had many versions over time, each better than the last. In this article we will take a look at how the periodic table evolved to its modern form.
A start by Alexandre-Emile Béguyer de Chancourtois.
Alexandre Béguyer de Chancourtois was a geologist, His principal contribution to chemistry was the 'vis tellurique' (telluric screw), a three-dimensional arrangement of the elements constituting an early form of the periodic classification, published in 1862.
The telluric screw plotted the atomic weights of the elements on the outside of a cylinder, so that one complete turn corresponded to an atomic weight increase of 16. As the diagram shows, this arrangement means that certain elements with similar properties appear in a vertical line. Although the telluric screw did not correctly display all the trends that were known at the time, de Chancourtois was the first to use a periodic arrangement of all of the known elements, showing that similar elements appear at periodic atom weights.
John Newlands' Law of Octaves
An English scientist called John Newlands put forward his Law of Octaves in 1864. He arranged all the elements known at the time into a table in order of relative atomic mass.
When he did this, he found a pattern among the early elements. The pattern showed that each element was similar to the element eight places ahead of it.
Newlands' table showed a repeating or periodic pattern of properties, but this pattern eventually broke down.
By ordering strictly according to atomic mass, Newlands was forced to put some elements into groups which did not match their chemical properties.
For example, he put iron (Fe), which is a metal, in the same group as oxygen (O) and sulfur (S), which are two non-metals.
As a result, his table was not accepted by other scientists.
Johann Dobereiner Law of Triads
Dobereiner’s triads were groups of elements with similar properties that were identified by the German chemist Johann Wolfgang Dobereiner. He observed that groups of three elements (triads) could be formed in which all the elements shared similar physical and chemical properties.
Dobereiner stated in his law of triads that the arithmetic mean of the atomic masses of the first and third element in a triad would be approximately equal to the atomic mass of the second element in that triad. He also suggested that this law could be extended for other quantifiable properties of elements, such as density.
The first of Dobereiner’s triads was identified in the year 1817 and was constituted by the alkaline earth metals calcium, strontium and barium.
Mendeleev's Periodic Table
In 1869, just five years after John Newlands put forward his Law of Octaves, a Russian chemist called Dmitri Mendeleev published a periodic table.
Mendeleev also arranged the elements known at the time in order of relative atomic mass, but he did some other things that made his table much more successful.
He realised that the physical and chemical properties of elements were related to their atomic mass in a repeating or 'periodic' way, and arranged them so that groups of elements with similar properties fell into vertical columns in his table.
Sometimes this method of arranging elements meant there were gaps in his horizontal rows or 'periods'. But instead of seeing this as a problem, Mendeleev thought it simply meant that the elements which belonged in the gaps had not yet been discovered.
He was also able to work out the atomic mass of the missing elements, and so predict their properties. And when they were discovered, Mendeleev turned out to be right.
For example, he predicted the properties of an undiscovered element that should fit below aluminium in his table. When this element, called gallium, was discovered in 1875, its properties were found to be close to Mendeleev's predictions. Two other predicted elements were later discovered, lending further credit to Mendeleev's table.
Henry Mosely's change to Mendeleev's Table
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).
Moseley determined the number of positive charges in the nucleus by measuring the wavelength of X–rays given off by certain metals in 1913. The wavelengths of the X–ray emissions of the elements coincided with the ordering of the elements by atomic number. When isotopes were discovered, it was clear that atomic weight was not as significant in periodic law, rather element properties varied with atomic number. When atoms were arranged by increasing atomic number, the problems with Mendeleev's periodic table disappeared.
Moseley revised the periodic table and made a bold change that removed all inconsistencies. He suggested that instead of arranging elements in the ascending order of their atomic masses, they should be arranged in the ascending order of their atomic numbers. The number of electrons in an atom is equal to the atomic number Z. Thus by making this change, Moseley put the emphasis on electronic configuration of the elements. Also it must be remembered that all physical and chemical properties come about because of the arrangements of electrons.
Conclusion
The Periodic Table was the combined effort of many scientists and took decades to reach its modern form. The modern periodic table is almost a perfect classification of the elements known to mankind. Using this table we have managed to successfully classify 118 elements already. Thank you for reading this article.
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