Periodic table a living document

If there is one thing most people associate with chemistry, it is the periodic table. After all, pretty much every science classroom in the world has one hanging on the wall.

It was 150 years ago that Dmitri Mendeleev first published his version of the table with all 56 known elements. It has undergone numerous modifications since then - there have been over 120 official versions in the intervening years - but the table owes its origins to much older chemistry. It is a living entity changing with time and new discoveries.

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It is also a subject of some controversies perhaps only a chemist could love.

Chemistry started with a few "native" elements - elements which can be found in natural settings. The most obvious of these is carbon which is found in soot. Indeed, our very ancient ancestors used soot as a component in their cave paintings along with other chemical compounds.

Gold, silver and copper can all be found in their metallic state. They are the coinage metals and have been used as such since at least the sixth century BCE to make money.

Iron can also be found as a metal in some deposits due to its presence in certain types of meteors but it can also be separated from sulfur by heating iron pyrite or fool's gold under the right conditions. Similarly, both mercury and lead were obtained millennia ago by heating their respective sulfides. And, of course, sulfur was a byproduct of these reactions.

It was also available in a nearly pure form from the fumaroles near hot springs and volcanic vents.

Along with tin, zinc, arsenic, antimony, and bismuth, these elements made up the ancient chemical toolbox and were the basis for much of the development of ancient civilization.

It wasn't until 1669 that a new element was added to list with the discovery of phosphorus by Hennig Brandt. Over the next 150 years, many more elements were discovered. Gases such as oxygen and nitrogen were isolated from the air while chlorine was synthesized using electrolysis. Metals ranging from titanium to iridium were separated and purified from their mineral ores.

By the mid-1800s, a total of 56 elements had been characterized.

Many early attempts were made to rationalize these elements into groupings. In 1829, Johan Wolfgang Dobereiner published his work on triad in which the elements had similar chemical properties and a mathematical relationship for their atomic weights. Chlorine, bromine and iodine make up just such a triad. Each reacts with the alkali metals in a one-to-one ratio and a two-to-one ratio with the alkaline earths.

Other metals, such as iron, result in compounds with either two-to-one or three-to-one ratios. And reactions with hydrogen generate acids. On top of this, their atomic weights are related as the lightest and heaviest average out to the middle element.

In 1860, at the Karlsruhe conference, Stanislao Cannizzaro proposed atomic weights as an organizing principle for the elements. Two years later, Alexandre-Emile Beguyer de Chancourtois published a listing of the elements, based on their atomic weights, which spiraled around a cylinder leading to the idea that the elements could be arranged in columns.

But it is Mendeleev's table which he published in 1869 which was the real beginning of the modern periodic system and our understanding of chemistry. Mendeleev left blanks in his table recognizing there were elements left to be discovered.

He built his groups - the columns we now see in the modern table - around common chemical and physical properties. For example, all of the elements in the carbon group form tetrahalides.

Perhaps the most important aspect of Mendeleev's table is he was able to make predictions about the missing elements such as their density, structure of their oxides, and reactivity. These were bold predictions but within a short period of time the predicted properties had allowed other scientists to find many of the missing elements.

Despite the success of the early periodic tables, they were many issues - not the least of which is the funny shape.

The elements do not organize into a nice, neat square and there are certain some properties which make it difficult to place elements.

Then there are the anomalies. For example, the atomic weight of tellurium is actually greater than its neighbour iodine despite being listed before it. However, Mendeleev noted the chemical properties of tellurium were much more in keeping with selenium, sulfur and oxygen than with the halogens and iodine was clearly related chemically to bromine and chlorine. It was his confidence in the chemistry which allowed Mendeleev to build his table.

Some 45 years later, Henry Moseley was able to show it was atomic number not weight which matter and validated Mendeleev's choice.

More on that next week.

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