The periodic table orders all the elements in neat rows and columns as a guide to their properties and atomic structures. Atomic structure, in fact, explains why the periodic table works, but this was understood much after the table was first constructed.
The backstory to the construction is no less fascinating. It is marked by the tireless efforts of scientists in difficult conditions, their failures and insightful predictions, and refinements made to the table as scientific knowledge expanded.
Why a table was made
To get a sense of its significance, it is necessary to first recognise the circumstances under which the periodic table was created in 1869. The number of known elements was growing, and scientists wanted to order them in a meaningful way. But no such order was immediately apparent, with each element coming with its own set of properties.
In 1862, a French geologist, Alexandre-Émile Béguyer de Chancourtois, found a pattern. At that time, scientists already knew the mass of an atom of each known element; although the technology did not exist to isolate and weigh the atoms, these atomic masses could be calculated by observing what amounts of one element combined with what amounts of another element to create a certain amount of a compound. De Chancourtois arranged the elements in order of their atomic masses, and noticed a “periodicity” in their chemical properties. In other words, the properties of one element would repeat themselves in another element after a certain “period” as one proceeded in order of atomic mass.
For example, lithium and sodium, separated by 16 places in the order of atomic masses, were chemically similar in many respects; both metals are highly reactive, and both release hydrogen gas when they react with water. The two elements could thus be placed in the same vertical column; so could potassium further down the list.
Such columns made for a more orderly arrangement than a single row, which would have gone on forever. In 1864, the British chemist John Newlands made the same observations. The real periodic table, however, was yet to come.
The genius of Mendeleev
In 1869, the Russian chemist Dmitri Mendeleev, along with the German chemist Lothar Meyer, proposed a tabulation of the elements which was different from previous arrangements in at least two ways.
In a stroke of genius, Mendeleev prioritised an element’s properties over its atomic mass. If the situation demanded it, he boldly interchanged the positions of elements. The classic example is of iodine (atomic mass 126.9) and tellurium (127.6). In terms of atomic mass, iodine should have come before tellurium, but Mendeleev chose to swap them, placing each one in a column that already had other elements with similar properties. Thus, tellurium went below selenium in one column, and iodine below bromine in the next column. Because selenium was already placed before bromine, this arrangement effectively brought the heavier tellurium before the lighter iodine.
The periodic table, therefore, was no longer strictly in the order of atomic mass. In the process, Mendeleev introduced the concept of “atomic numbers”, assigning these numbers to reflect the position of each element in the periodic table.
Mendeleev’s other innovation was no less bold. He predicted the existence of elements that had not yet been discovered. According to the positions he had assigned, there should have been elements with certain properties with the atomic numbers 23, 31 and 32. Since no such elements were known, he left those positions blank in the periodic table, confident that these elements must exist. As it turned out, all three elements were discovered during his lifetime — gallium (31) in 1875, scandium (23) in 1879, and germanium (32) in 1886.
Many more new elements would follow over the next few decades.
Filling up the gaps
The hunt for new elements coincided with a series of advancements in scientific knowledge. Radioactivity and X-rays, both discovered in the 1890s, led to the detection of a number of elements.
The periodic table, which reflected atomic mass, did not undergo any change as a result of the discovery of the electron. However, the electron finally explained the mystery behind the neat patterns in the periodic table which even Mendeleev did not know of when he introduced the atomic numbers on the basis of chemical properties alone.
Two discoveries using radioactivity are now the subject of scientific lore. In 1898, Marie Curie and her husband Pierre found that the radioactivity from pitchblende, a uranium-rich mineral, was so high that the uranium alone could not explain it, implying it had at least one other radioactive element.
The events that followed have been written about several times, and are depicted in a simplistic, accessible manner in the film Madame Curie (1943), which has Greer Garson and Walter Pidgeon playing the Curies. Using very basic equipment in a shed, they experimented on tonnes of pitchblende and isolated polonium (atomic number 84), followed by radium (atomic number 88).
X-rays came into the picture in 1914, when the English physicist Henry Mosley measured the wavelengths of X-rays scattered from different metals. As he went from one element to another higher up the periodic table, the wavelengths decreased successively, at a predictable rate.
This knowledge, in turn, allowed scientists to predict elements missing in the periodic table. For example, if elements A and B were next to each other in the periodic table, and produced X-rays whose wavelengths differed by exactly the predicted amount, then they must be in their right places. But if the difference did not match the prediction, then there needed to be one or more elements in between. By comparing the actual difference with the predicted difference, it was also possible to calculate how many elements were missing.
The periodic table could thus be refined. Scientists identified a number of empty slots between atomic numbers 1 (hydrogen) to 92 (uranium). One by one, all these slots were filled by 1945, mostly with radioactive elements.
But by then, the hunt had already extended to elements beyond uranium at 92. In due course, a number of trans-uranium elements were detected by means of bombardment with particles, which resulted in creating atoms of the next higher element on the periodic table.
Today, the periodic table stands at 118 elements, each in its rightful place, and with no gaps in between.
Behind the pattern
What is it about atomic numbers that dictates the elements’ properties and, consequently, their positions in the periodic table? At the time Mendeleev had assigned these numbers, no one knew, because the structure of the atom was not yet fully understood.
Enlightenment came with the unearthing of the electron (first detected in 1854, described in detail in 1897). Subsequent studies showed that the atomic number of each element was the same as the number of electrons in its atom.
Then in 1925, the Austrian physicist Wolfgang Pauli showed how electrons are distributed among a number of shells, something dictated by simple mathematical rules. As it turned out, an element’s chemical properties depend on the number of electrons in the outermost shell of its atom.
The positions in the periodic table were, therefore, dictated by these so-called valence electrons. The mystery had finally been solved.