The story of the periodic table, which as of December 2015 has been completed with the discovery of the final four predicted elements, is one of the great triumphs of the scientific method.
The concept of indivisible particles that are the fundamental building blocks of all matter (atoms) has been around since ancient times.
In 360 BC, the Greek philosopher Plato coined the term “elements,” although he was far off base with the idea that the elemental particles were earth, air, fire and water. These are more analogous to the modern states of matter, solid, gas, plasma and liquid.
Certain elements that occur naturally in their pure form including sulphur, silver, gold, copper, iron, lead, tin, mercury and carbon have been known for millennia.
It was not until the 1600s, however, that scientists started developing an understanding of true elemental particles and identifying these substances and others that had been discovered starting in the 1200s AD as elements.
By 1778, there were 33 known elements when Antoine Lavoisier, a French nobleman and chemist made the first attempt to classify the elements based on their properties by separating them into gases, metals, non-metals and earths. This was perhaps the first contribution to the periodic table as we know it today.
Others also noticed patterns. In 1829, Johann Döbereiner recognized triads of elements with similar properties such as lithium sodium and potassium demonstrating the properties of the middle one could be predicted by the other two.
In the 1860s, British chemist John Newlands was the first to describe the periodicity of elements noting similarities between elements that had atomic weights that differed by seven and called it the Law of Octaves.
It was on this foundation that Russian Dmitri Mendeleev produced the earliest periodic table in 1869. Legend has it he made the discovery on a train using a card for each of the known elements on which he had written their properties. By arranging and rearranging them he realized if he put them in order of increasing atomic weight regularly occurring patterns emerged. Initially, he had them arranged in horizontal rows, but quickly realized the utility of placing them in columns.
Mendeleev’s effort was not an exact replica of the modern table, but everything that was to come, fit into the basic model he developed.
For example, Mendeleev’s table (and Newland’s Law of Octaves) was lacking the noble gases (helium, neon, argon etc.), none of which had been discovered at the time. When they were, however, they fit perfectly as the eighth column in the table as we know it today.
There were other problems because nature rarely perfectly conforms to the human need for order. For example, the atomic weight of tellurium is greater than that of iodine, but by their properties they did not fit. Mendeleev correctly transposed them.
The true, crowning achievement of Mendeleev’s work, however, was that he left gaps for undiscovered elements even predicting the properties of five of them.
Within 15 years, three of his predicted elements had been discovered and his predictions proved extremely accurate.
For example, Mendeleev predicted an element he called eka-aluminum—eka, from the latin meaning assumed to stand next in order—because there appeared to be one missing between aluminum and indium in column three. He said this element would have an atomic weight of approximately 68, its density as a solid would be six grams per centimetre cubed, its melting point would be low, its valency would be 3, it would be discovered by its spectrum, its formula would be Ea2O3 and would be soluble in both acids and alkalis.
When French chemist Paul-Émile Lecoq de Boisbaudran discovered by spectroscopy what we now know as Gallium, it almost perfectly matched those predictions greatly enhancing the reputation of Mendeleev’s periodic table.
It would be refined over the first half of the 20th century to the form we now know with its eight groups (columns), seven periods (rows), four sub-rows (the transition metals including the separate Lanthanide and Actinide series that flesh out periods six and seven.
In the latter half of the 20th century most of the gaps were filled in with the exception of four super heavy elements at the end of the seventh period, numbers 113, 115, 117 and 118.
Finally, at the end of 2015, the final four spaces were filled in, 115, 117 and 118 by a joint Russo-American venture and 113 by a team from Japan. The achievements was confirmed by the International Union of Pure and Applied Chemistry (IUPAC) at the end of December.
The completed table has 118 elements, 94 of which have been found occurring naturally. Of these 94, 10 only exist as temporary products of radioactive decay. The other 24, with atomic numbers 95 through 118 (everything above plutonium) have only ever been synthesized in a laboratory, the problem being that super heavy elements are highly unstable and decay extremely rapidly.
So, what is next for chemistry? The great thing about science is it doesn’t stop at complete.
The Irish comedian Dara O’Briain does a great bit on pseudoscience talking about how people peddling nonsense frequently use the argument, ‘well, science doesn’t know everything.’ O’Briain’s response: “Science knows it doesn’t know everything, otherwise it would just stop.”
Theoretically, there could be an eighth period, possibly even a ninth and tenth. It has even been hypothesized that within the eighth period there could be an “island of stability” a group of elements that are both super heavy and stable.
Kosuke Morita, who led the Japanese researchers who discovered element 113, said his team will now “look to the uncharted territory of element 119 and beyond.”
