Chemistry Unit 3 Topic 4

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C3-4 : Development of the Periodic Table

The Earlier Table of the Elements
( of the first proper stabs at developing a table of the elements was by John Dalton. By testing the known elements’ reactions he arranged them all in order of mass. The image shows Dalton’s finalised table [click on the image to see the full size table]. As different as it looks to the modern table, it actually bares some resemblance! John Newlands built on Dalton’s table using the law of octaves, suggesting that every eight element had similar properties (now you can see the similarity).

The problem with Newlands’ table was that he was too determined to get it done and working that he made some mistakes. What he didn’t know was that there were still many elements to be found, so he filled in octaves regardless of their properties, and some of them ended out not being similar at all. He did this to make everything fit in, so as a result, his ideas were not accepted.

On the bright side, a year prior to Newlands’ attempt, the French chemist Alexandre-Emile Beguyer de Chancourtois had a better go at arranging the elements by ordering them by properties (in sections of eight again). He successfully made a very clever diagram table of the elements. Unfortunately, the diagram was missed out when his work was published!

( the late 1860s, things started to look better though, because the Russian scientist Dmitri Mendeleev created a very promising table. By this time, fifty elements had been discovered, and Mendeleev arranged them all in order of atomic mass. After that, he arranged them into further groups based on their behaviour and properties, to arrive with aperiodic table. He is considered to be “the father of the modern periodic table.” [Click on the image to see his enlarged table] A minor problem with this table was that there had to be a few blank spaces – although to be fair at least he recognised that certain elements weren’t discovered by looking at his table – he didn’t try to cram them all in like Newlands.

The Modern Periodic Table of the Elements
A problem faced by Mendeleev was that although arranging elements in order of atomic mass produced groups of elements which behaved the same way, not all elements did do this. For example (look at a modern table for help here), argon (Ar) has a higher atomic mass than potassium (K) and so would be placed after potassium with the reactive metals – but argon is a noble gas! Therefore, argon was put before potassium, even if the argon atom was heavier.

In the early 20th sentury, scientists began to look more closely at the atomic structure and decided that the way to solve problems like the one described above was to arrange all the elements in order of atomic number (or proton number). This put them in exactly the right…


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