For thousands of years some of the greatest minds in history have attempted to classify all of the known substances in existence. Although many great thinkers had attempted this feat no one was able to come up with a satisfactory explanation and organization for the substances all around them. The struggle continued until 1803 when chemist John Dalton was the first person to experimentally prove the atomic nature of elements. His theory stated that atoms were indivisible building blocks of matter on which everything was built. These atoms made up a few basic substances called elements on which all matter is built. Although we later found that these atoms were divisible, Dalton’s theory set up the race to classify these basic elements.
In 1829 the German chemist Johann Wolfgang Dobereiner was the first person to experimentally express the idea of grouping elements according to repeating trends in their properties. Many elements, he noticed, fit into similar groups of three. He called these groups triads. He would later observe that when arranged according to atomic weight the second element was nearly the average of the first and third. The later became known as the law of triads.
As chemists began working with the idea of Dobereiner’s triads they began to discover even larger groups of elements, often similar to triads but containing more then three elements. In 1864 German chemist Julius Lothar Meyer published a paper combining the observations of some of his contemporaries with his own and arranged elements based off of the ratios they tended to combine with others. This idea came to be known as valency and is the basis for modern ideas of bonding. Elements with the same valency tended to have similar properties and were precursors to the groups (columns) of the current periodic table.
Also in 1864 Englishman, John Newlands arranged the elements according to atomic weight. He noticed that in this order it seemed that some chemical properties repeated every eight elements. He likened this to the idea of musical octaves. Although his work did not gain mainstream acceptance at the time it is similar to the modern concept of the octet rule for valance electrons, or those in the outermost energy level off an atom. This rule is largely the basis for the modern idea of bonding. When combined with Meyer’s ideas of valency these ideas were over fifty years ahead of their time.
The biggest step in the quest to classify the chemical elements came soon after in 1869. In that year Russian chemistry teacher Dmitri Mendeleev published his periodic table of the elements. He organized the known chemical elements in order of increasing atomic weight. When an element’s properties reflected that of another he started a new row, placing it under the similar element. He made a few notable exceptions ot this process which turned out to be remarkably important in validating his work to later generations.
The first of these choices was to leave some blank spaces in the table where it seemed an element with the correct properties was not known. It turned out that as more elements were discovered they fit correctly into these slots, showing just how accurate and intuitive Mendeleev’s design actually was. He took it one step further by correctly predicting the properties of many of these elements. These correct predictions further validated the quality of his table and helped it to gain widespread acceptance.
The second choice he made was to occasionally ignore the atomic weight when the properties of the elements seemed to fit better in another order. It eventually became known that what he had done was actually place the elements in order of atomic number. Since subatomic particles were still undiscovered he did not know this, but it gave his work even more credibility with future scientists.
By 1913 the work of Ernest Rutherford and Henry Mosley showed that atoms were really made up of positively and negatively charged particles known as protons and electrons. It soon became apparent that the number of these particles was the key factor in the periodicity of the elements, rather then atomic weight as was previously accepted. With the work of the great Danish physicist Neils Bohr in the 1920’s and 1930’s the world came to better understand the reasons behind the long observed periodic trends of the elements.
The work of these scientists led to the discovery of electrons, and quantum theory. Scientists now knew that elements had shells or levels of electrons, and that they bonded in order to fill these shells with eight valance electrons. This validated many of the ideas of the past, including the ideas of valance and octaves, but also required a few minor tweaks to the peirodic table.
Using this information it was now possible to add labels to the rows and columns of the table, denoting the energy levels of the rows and valance electron numbers for the columns. As the properties of the elements became more easily tested and recorded people continued to add properties such as melting and boiling points, structures of the atoms.
As recently as the 1950’s the periodic table has undergone major additions. In 1951 Glenn Seaborg discovered several new elements and had to expand the table to accommodate them. He added a new period called the actinides under the lanthanides at the bottom of the table.
Even today the tables structure leaves room for continuous evolution. For almost 150 years it has evolved to meet the needs of the scientific community of the times. It has become the staple for professional chemists and young chemistry students alike. In fact the table is already able to accommodate a theoretical groups of elements called the g block, referring to the theoretical new energy sub level they will likely have. As we move into the new era of science the periodic table of the elements will be coming with us, accommodating our scientific needs every step of the way.
