The Periodic Table of the Elements is a display of the 118 known chemical elements. It is organized according to the repeating properties of the elements in each group on the table, hence the name periodic. Classifying these elements has been an ongoing journey for chemists and physicists ever since John Dalton discovered the atomic nature of elements in 1804. He learned that some substances were the indivisible building blocks on which other substances are formed, and thus began the journey to find and organize all of these substance, the elements.
The first periodic elemental classification system came along in 1829. A German chemist by the name of Johann Wolfgang Dobereiner observed that many of the chemical elements fit into groups of three called triads. These triads had similar chemical properties. An example is the trio of Lithium, Sodium, and Potassium. They were grouped together because they were soft and highly reactive metals.
He also observed that when his triads were arranged according to weight, the second element was the average of the first and the third. This came to be know as the law of triads. Dobereiner’s triads were the precursor to modern periods or families (columns on the periodic table) and represented well the elements known at the time.
As many chemists worked with Dobereiner’s triads more groups of elements were found, including some groups larger then three. In 1864 German chemist Julius Lothar Meyer published a paper combining the observations of many of his contemporaries, that arranged the elements based off of the ratio at which they tended to combine with other elements. It was observed that each element tended to combine with others in the same ratio, for example 1 carbon atom often combined with four methane atoms. This concept was know as valency. When arranged by increasing valency Meyer discovered that elements with the same valency had similar properties. This was the precursor to the modern idea of electron configuration that would not come along for over fifty years.
Also in 1864 John Newlands, a chemist from England observed that if he arranged the elements in order of increasing atomic weight their properties would repeat every eight elements. This was similar, he thought, to musical octaves. Although his work was not accepted as relevant at the time it is worth noting because it is the precursor to octet rule currently used to describe chemical bonding. Although electrons were not yet discovered at the time what Newlands was observing was really the number of electrons in the elements valance (outermost) electron shell which is why the modern table has eight main groups (called the representative elements).
Although the work of Meyer and Newlands seemed insignificant at the time it is notable how much ahead of their time they were before the advent of subatomic theory. They noticed trends that could not be explained until many years later. Their observation combined to make up the essence of atomic bonding theory even today.
The most significant step in the development of the periodic table of the elements occurred in 1869. This is when the Russian chemistry teacher Dmitri Mendeleev published his periodic table. He organized the elements in rows by atomic weight and started a new row when the elements’ properties repeated those of another.
He made two significant exceptions to this process that later turned out to be key in validating his work. First he left empty spaces in the table whenever an element seemed to be missing because its properties did not match with the element directly above. As new elements were discovered they fit into these empty spaces. Even more remarkably Dmitri was able to use the organization of the table to correctly predict the properties of these elements before they were discovered. This gave his work obvious credit with the next generation of scientists.
The second correct choice was to switch elements in certain situations, ignoring their atomic weight and instead placing them according to their repeating characteristics. This was correct because later it was found that he had actually arranged the elements by correct atomic number, or the number of protons (or electrons) the element had. This again showed later scientists that he had the correct idea when he published his table and it soon became accepted as the standard.
Of course some changes to the periodic table were later made based off of the discovery of subatomic particles and atomic number. The work of Ernest Rutherford in 1911 and Henry Moseley in 1913 showed that atoms were made up of positively and negatively charged parts called protons and electrons. It soon became obvious hat the number of these particles was the key factor in the periodicity of the elements rather then atomic weight.
With the work of the great Danish physicist Neils Bohr in in the 1920s and 1930s we began to better understand why period trends exist. He showed that elements have shells of electrons and that they need eight electrons to fill their outer shells. This lead to the development of quantum theory and validated the octave rule and valency properties observed by Meyer and Newlands. Now that it was understood why the trends acted as they did some tweaking was able to be done to the table to incorporate new elements. Some notation was added to the table to denote the number of valance electrons and electron energy levels of the periods and groups based off of Bohr’s discovery.
The last notable changes were made to the table by American Glenn Seaborg. He discovered a series of new elements that did not fit into the table automatically. To fit his new elements, called actinides, he extended the table and added them below the the lanthanide series. This extension made room for all of the synthetic and transuranic elements discovered since.
The table defines the elements in such a way that it is used to explain their properties in every discipline of modern science .It is designed so that it can carry us into the future as well. Its methodology leaves room for the many undiscovered and theoretical elements we may uncover in the future. In this way it truly is a timeless contribution to human understanding and an ever evolving one at that.
