Elementary order

How did elements come to be tabulated according to their properties?

 
By S Sankarapandi
Published: Sunday 07 June 2015

Elementary order

A COMMITTEE of the International Union of Pure and Applied Chemistry (IUPAC) -- the world body that finalises names of chemical substances -- recently abandoned a move to name an element after the American Glenn T Seaborg, who won the 1951 Nobel Prize for chemistry. The only reason given for the decision: Seaborg is still alive.

The 106th element -- an artificially generated element that disappears rapidly by emitting radiation -- was named seaborgium in 1993 by the American Chemical Society. But the Commission on Nomenclature of Inorganic Chemistry (CNIC) of the IUPAC recommended the name rutherfordium after Ernest Rutherford, the New Zealand physicist who established the structure of the atom.

However, Seaborg's contribution to chemistry is no less significant: the 82-year-old Berkeley scientist has discovered 9 elements encompassing atomic numbers 94-102, of which plutonium (94) is used in power generation. Albert Ghiorso of the Lawrence Berkeley Laboratory in California, the leader of the team that discovered element 106, comments on the IUPAC decision: "Seaborgium has fallen a victim to political horsetrading. It is unanimous in the scientific community that seaborgium is an excellent name. We'll do our best to see this overturned."

The cnic also recommended changes, which have been effected, in the provisional names assigned to elements of atomic numbers 101-109. The controversy over the naming of new elements has been given a new twist with the creation of an element of atomic number 110 by a German team led by Sigurd Hofman. They refuse to propose a name for it to protest the way IUPAC has named the elements 101-109.

But whatever the names that are finally ascribed to the elements in question, once they are isolated they are assured a slot in the Periodic Table. The table was evolved to fulfil a long-felt need to categorise the elements in such a way that those with similar properties appeared together. But the Periodic Table, in which elements with similar characteristics appear in the same column, was the culmination of years of scientific thought.

Elements are the fundamental substances from which all material objects in the world are formed. In the mid-17th century, Robert Boyle, an Anglo-Irish chemist, first scientifically defined an element: a substance that cannot be broken down into simpler constituents and which combines with other elements to form compounds. Boyle's definition was confirmed by the discovery of 2 gaseous elements by 2 English scientists -- hydrogen by Henry Cavendish, and oxygen by Joseph Priestley -- in the 2nd half of the 18th century. Around the same time, French chemist Antoine Lavoisier showed that water was a compound of hydrogen and oxygen.

By the turn of 18th century, only a handful of elements had been identified. Volta's cell, invented by Italian physicist Alessandro Volta in 1800, provided a powerful method of separating more elements like sodium and potassium from their natural compounds by using an electric current. The problem now was how to represent these elements with symbols. Jons Berzelius, a Swedish scientist, suggested in 1813 that elements can be denoted by abbreviating their names. This became the basis of chemical symbols. For example, oxygen and carbon are represented by their respective abbreviations O and C.
Signature of an element With the discovery of new elements, scientists started investigating the differences between them. In the early 19th century, John Dalton, an English scientist, propounded the atomic theory. He suggested that all elements were composed of minute indivisible particles called atoms and that atoms of one element are distinct from those of another. Atomic weight -- a specific characteristic of an element -- became the signature of an element. Many scientists -- John Dalton of England, Gay-Lussac of France, Amedeo Avogadro of Italy, Jons Berzelius of Sweden -- plunged into determining atomic weights using different methods.

Also, chemists had observed that some elements had similar properties and they felt the need to classify them in order to arrive at a correlation between them.

German chemist Johann Dobereiner's triad grouping in 1817 was the first attempt at classifying elements. He showed that elements with similar properties make a triad in which the atomic weight of an element lies midway between those of the other 2. By 1843, he and a few other scientists had identified at least 10 triads, while some others expanded it further to tetrads with 4 elements, and pentads with 5 elements. However, this method proved unsuccessful because chemists arrived at conflicting values of atomic weights for some elements.

To resolve the issue, chemists decided to get together and the first International Congress of Chemists was held at Karlsruhe in Germany on September 3, 1860, where atomic weight values were revised.

In 1862, French geologist Begauyer Chancourtois proposed a new classification method. He plotted atomic weights on the surface of a cylinder with a circumference of 16 units -- the approximate atomic weight of oxygen -- and arranged the elements in a line that descended at an angle of 45 degrees from the top of the cylinder. The resulting helical curve brought similar elements onto corresponding points above or below one another on the cylinder. Although it was the first periodic classification, this method did not work for all elements.

In 1864, chemist John Newlands noticed a periodicity existing among elements when arranged in ascending order of their atomic weights. He found that every 8th element exhibited properties similar to the 1st and called the system "law of octaves", an analogy with octaves in music. But Newlands' paper was rejected and ridiculed by the Chemical Society, London. Only 24 years later did the Royal Society recognise Newlands' contribution and awarded him the prestigious Davy medal.

But it was Russian chemist Dmitri Mendeleyev who is credited with the discovery of the Periodic Table. In 1869, he proposed the Periodic Law: elements arranged in an ascending order of their atomic weights show a periodic change of properties. His first table contained 17 vertical columns with 2 complete horizontal rows, called periods, of elements along with 2 partially complete periods. Lothar Meyer, a German chemist, simultaneously working on periodic classification, published a similar table in 1870, but with some variations.

Mendeleyev and Meyer together proposed a revised table by splitting each of the long periods with 17 columns into 2 periods of 7 columns -- Groups Ia to VIIa and Ib to VIIb -- and also an 8th column, VIII. In 1894, 2 British scientists, Lord Rayleigh and William Ramsay, discovered a gaseous element, inert to chemical reactions and called it Argon -- "the lazy one". Subsequently, 4 more similar elements were identified; their inertness posed problems in determining the atomic weights by the usual methods. Mendeleyev and others put these inert gases under a new group called the "zero" group in the Periodic Table.

The greatness of Mendeleyev's periodic law was its ability to predict the existence of the then undiscovered elements. For instance, when Mendeleyev made his 1st table, the elements gallium and germanium were unknown. He left gaps in his table for these elements since he anticipated their existence. Later, when these elements were discovered, they fit precisely in the same gaps.

The table enabled scientists to correct the errors in the atomic weights determined earlier. Mendeleyev's table -- which was formulated when the structure of atom was not understood -- also predicted many other properties like the density and melting point of known and unknown elements from the corresponding properties of the nearby elements on the table. Mendeleyev was honoured for his discovery in 1955 when the 101st element was named mendelevium after him.

Structure unveiled
However, the positions of some elements in Mendeleyev and Meyer's periodic table -- if fixed by the comparison of properties with other elements -- did not tally with their ascending order of atomic weights. This discrepancy was understood and sorted out only when the structure of the atom was unveiled in the '30s by Niels Bohr, a Danish physicist, Ernest Rutherford and the Austrian physicist, Wolfgang Pauli.

Thanks to these scientists, it was known that the nucleus of an atom consists of positively charged particles, called protons, along with neutral particles, or neutrons. The nucleus is surrounded by negatively charged electrons, whose number is equal to the number of protons inside the nucleus. Moreover, scientists realised that electrons rather than protons or neutrons play a major role in defining the properties of elements.

This discovery led German scientist Van den Brock to suggest that atomic number -- the number of electrons or protons in the atom -- would be the more appropriate identity of an element than its atomic weight, which is the sum of the number of protons and neutrons. This suggestion was confirmed by experiments conducted by English physicist Henry Moseley. He showed that the interaction of elements with X-rays -- that differentiate the elements -- was dependent on the atomic number, not the atomic weight. The modern Periodic Table -- based on the ascending order of atomic numbers -- thus became the perfect Periodic Table.

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