150 years of periodic table: it’s elementary!

This year has been declared the ‘International Year of the Periodic Table’. It commemorates the seminal work in chemistry that led to the periodic table of chemical elements created 150 years ago, in March 1869, by Dmitri Ivanovich Mendeleev.

The periodic table put known chemical elements into a logical order, leaving room for elements yet to be discovered. The table marked the transformation of chemical science from the medieval magical mysticism of alchemy to the realm of modern scientific systematic rigour. Indeed, it is to chemistry what Newton’s Principia (1687) is to physics (laws of motion, gravity etc), On the Origin of the Species (1859) to biology, or Copernicus’ treatise on the heliocentric theory (1543) is to astronomy.

The Mendeleev table uncovered a deep truth, the so called periodic law. As enunciated by Mendeleev, the periodic law is the principle that the physical and chemical properties of elements are a periodic function of their atomic number (or atomic masses as used in the initial table). It even enabled him to predict the existence of elements not yet discovered.

Apart from predicting new elements, it provided support to the then controversial belief in the reality of atoms, even hinting at sub-atomic structure and anticipating underlying rules governing atoms. Eventually, all this was explained 50 years later in quantum theory, involving electronic shell structure and the Pauli Exclusion Principle.

The periodic table of 1869 was a vertical chart that organised the 63 elements known then by atomic weight. This arrangement placed elements with similar properties in horizontal rows. The title translated from Russian read “Draft of a system of elements based on their atomic masses and chemical characteristics.” The table was a culmination of earlier efforts by chemists like John Dalton, Johann Döbereiner, John Newland, etc.

Mendeleev left blank spaces in his original table so that he could properly line up the known elements. Gallium, element 31, was the first gap to be filled with its discovery in 1875. It had an atomic weight of about 68 and a density of 6. Mendeleev predicted just these values six years before. He called it eka-aluminium (element 13). He also predicted for eka-silicon, an atomic weight of 72, which matched that of Germanium (72.3). Gallium is in the same group as Aluminium and Germanium is in the Silicon group. Scandium, which he also predicted, was discovered within his lifetime with the observed properties.

The elements fall into vertical columns called groups. Going down a group, the atoms of the elements all have the same outer electron shell structure, but an increasing number of inner shells. Modern practice is to number the groups across the table from 1 to 18. One corresponds to the alkali metals (Lithium, Sodium, Potassium, etc) while 18 corresponds to the noble (inert) gases such as Helium, Neon etc. None of the noble gases were known to Mendeleev. Helium was identified by its spectral line in the solar atmosphere. Neon, Argon, Xenon, etc were identified by Ramsey in the fractional distillation of liquefied air.

Horizontal rows in the table are periods. First three are called short periods, while the next four (which include the transition elements) are called long periods. Within a period, as stated above, all elements have the same number of shells, but with a steadily increasing number of electrons in the outer shells. The table can be divided into four blocks depending on the type of shell being filled, the S, P, D and F blocks.

Mendeleev had no idea of electronic shells or of electrons or protons. That is why he arranged his table in order of increasing atomic weights. Now we know that it is the atomic number, i.e. the number of electrons in the particular atom (orbiting the nucleus) or the number of protons in its nucleus, which determines the chemical properties. The reason why properties of elements are similar for every 8th or 18th or 32nd element is because the electronic P shell can accommodate only 6 electrons, the D shell 18 and F shell 32. The S shell can accommodate 2 electrons. This is elucidated in the Pauli Exclusion Principle.

Transuranic elements

Element number 118, also an inert gas, has just been discovered and named Oganesson, after Yuri Oganesson, the Russian chemist who pioneered the discovery of new transuranic elements in the Dubna laboratory in Russia. This is the last element in the periodic table. The few atoms of this element made, have survived for less than a millisecond.

Just a few months ago four new elements, including No 118, were added to the periodic table. These are Nihonium (113) named after Japan, Moscovium (115) after Moscow and Tennessium (116). Element 112 named Copernicium (only element named after an astronomer) is expected to be similar to Mercury (difference in atomic numbers is 32). It turns out to be a conducting gas. Uranium 92 is the last element on the table that occurs in any meaningful quantity.

Heavier atoms have been created in the laboratory and are known as transuranic elements. Glen Seaborg was a pioneer in the production of transuranic elements in reactors. The modern form of the periodic table, a horizontal design, in contrast to Mendeleev’s original version became popular largely due to Seaborg. He saw the need for a new row in the table for these transuranics.

This new table added a row for these elements beneath a similar row for the rare earth elements — starting from Lanthanum (57) to Lutetium (71). The unstable elements Technitium (43) and Promethium (61) were also made first in nuclear reactors and later identified in the atmosphere of some evolved stars by astrophysicists, proving that elements are being synthesised by nuclear reactions in these stars. Search is now on for elements beyond 118.

There are basic questions like whether there is an upper limit to the atomic number, i.e. number of elements. Is it 137 or 170? Can such elements be produced in nature and what would be their exotic properties? In short, Mendeleev’s periodic table has inspired generations of chemists to add more and more elements to it and the search to find heavier elements continues.

(The author is with Indian Institute of Astrophysics, Bengaluru)