Plenty of chemists in the 1860s noticed that the elements fall into repeating patterns. Several published tables. What separates Dmitri Mendeleev from all of them is that he trusted his arrangement more than he trusted the data, left holes where he believed elements were missing, and described in detail what those elements would be like when somebody eventually found them.
Within seventeen years, three of them had been found, and each matched his description closely enough to settle the argument permanently.
What Had to Happen First
A periodic table is impossible without reliable atomic weights, and for most of the nineteenth century chemistry did not have them.
The problem went back to Dalton’s atomic theory of 1808. Dalton could measure the proportions by mass in which elements combine, but not how many atoms of each were involved, so he guessed the formulas and his weights inherited the errors. Different laboratories used different assumptions, and the same compound might be written three ways in three countries.
The situation was resolved at the Karlsruhe Congress of 1860, where Stanislao Cannizzaro revived Avogadro’s hypothesis and showed how to derive consistent atomic weights from gas volumes. Mendeleev, then twenty six, attended. He later described the meeting as decisive for his thinking. It gave him the one thing an ordering scheme requires: a set of numbers everyone could agree on.
The Attempts Before Him
Mendeleev was not working in a vacuum, and it is worth being clear about what others had already done.
Johann Döbereiner had noticed as early as 1817 that certain elements fall into triads, where the middle member’s atomic weight is roughly the average of the other two. John Newlands proposed a law of octaves in 1865, observing that properties seemed to repeat every eighth element when listed by weight. He was mocked at the Chemical Society, where one member asked whether he had tried arranging them alphabetically.
Most significantly, Julius Lothar Meyer in Germany produced a table very similar to Mendeleev’s at almost the same time, based on the periodicity of atomic volume, and published in 1870.
Meyer’s work was excellent and he shared the Davy Medal with Mendeleev in 1882. The difference between them is the one that mattered: Meyer arranged the elements that were known. Mendeleev used the arrangement to make claims about elements that were not.
The Table of 1869
Mendeleev was writing a textbook, Principles of Chemistry, and needed a rational order in which to present the sixty three elements then known. He wrote the properties of each on a card and spent a long time rearranging them, a process he later compared to a game of patience.
The arrangement he arrived at in February 1869 sorted the elements by increasing atomic weight, with a new row begun whenever the chemical properties started to repeat, so that similar elements fell into vertical columns. Lithium, sodium and potassium sat together; fluorine, chlorine and bromine sat together.
He then did two things that his contemporaries were not willing to do.
He left gaps. Where the next element in weight order had the wrong properties for the position, Mendeleev assumed an undiscovered element belonged there and moved on, leaving the space empty.
He overruled the measurements. In several places the atomic weights put elements in positions that contradicted their chemistry. Mendeleev placed them according to their properties and declared that the accepted weights must be wrong. In some cases, including beryllium and indium, he was right and the weights were later corrected. In the case of tellurium and iodine, where he swapped the order against the weights, the weights were correct and the underlying reason for the anomaly would not be understood for another forty years.
The Predictions
The gaps are what made the table a scientific theory rather than a filing system. For three of them Mendeleev published detailed forecasts, naming the missing elements provisionally with the Sanskrit prefix eka, meaning one, so that eka-silicon meant the element one place below silicon.
He predicted atomic weight, density, melting behaviour, the formula of the oxide, the formula of the chloride, and in some cases the colour and the method by which the element would most likely be isolated.
- Eka-aluminium was found in 1875 by Lecoq de Boisbaudran, who named it gallium. Mendeleev, reading the announcement in Russia, wrote to point out that the reported density was wrong. Boisbaudran remeasured it with a purer sample and found that Mendeleev, who had never seen the substance, was correct
- Eka-boron was found in 1879 by Lars Fredrik Nilson and named scandium
- Eka-silicon was found in 1886 by Clemens Winkler and named germanium. The predicted atomic weight was 72 against an actual 72.6, and the predicted density 5.5 against an actual 5.35
Predicting the properties of a substance that nobody has ever seen, from its position in a table, is the kind of result that converts sceptics. After germanium, the periodic law was no longer one arrangement among several.
What Mendeleev Did Not Foresee
Two developments caught him out, and both are instructive.
The first was the noble gases. Between 1894 and 1898, William Ramsay and Lord Rayleigh isolated argon, helium, neon, krypton and xenon: a family of unreactive elements for which the table had no gap at all. Mendeleev was initially sceptical and at one point suggested argon might be a form of nitrogen. The solution was to add an entire new column at the edge of the table, which the structure accommodated without difficulty. A theory that can absorb a whole unexpected family is in better shape than one that has to be abandoned.
The second was the explanation of why the periodicity exists at all. Mendeleev ordered by atomic weight because that was the only quantitative property available. In 1913 Henry Moseley measured the X-ray spectra of the elements and found that a simple relationship connects the spectral frequency to a whole number, one that increases by exactly one from element to element. That number is the positive charge on the nucleus Rutherford had identified two years earlier.
Ordering by atomic number rather than atomic weight removed every remaining anomaly at a stroke, including tellurium and iodine, where the heavier element genuinely does come first. Moseley also showed how many gaps remained to be filled, and reduced the search for new elements to a bounded problem. He was killed at Gallipoli in 1915, aged twenty seven.
The deeper explanation followed from atomic structure. Chemical behaviour depends on the arrangement of electrons, and Bohr’s model of energy levels combined with the Pauli exclusion principle accounts for exactly why the rows have the lengths they do. The periodic table turned out to be a map of electron shells, drawn half a century before anyone knew electrons existed.
The Man
Mendeleev was the youngest of a large Siberian family, brought to St Petersburg by his widowed mother so that he could study. He was a serious chemist across many fields, working on the solubility of alcohol in water, on petroleum, and on the metrology of the Russian state as director of the Bureau of Weights and Measures.
He was nominated for the Nobel Prize in Chemistry in 1906 and lost by a single vote in the committee, partly because of the argument over Lothar Meyer’s priority. He died in 1907. Element 101, mendelevium, was named for him in 1955.
Why It Still Hangs on the Wall
The periodic table is the only diagram found in essentially every chemistry classroom in the world, and its longevity comes from doing two jobs at once.
It organises what is known, compressing an enormous quantity of chemical fact into a single sheet. And it makes claims about what is not yet known, since a position in the table implies a set of properties whether or not anyone has ever held the substance in their hand. That second function is what made it a genuine scientific law rather than a convenient chart.
The physics that eventually explained it grew out of the same period, and our edition of Max Planck’s three key publications collects the works where the quantum ideas behind electron shells were first set out, by the man who introduced them with considerable reluctance.
No products in the cart. 