It’s a rarity that we should come across a laboratory, classroom, chemistry textbook or lecture theatre that doesn’t contain a periodic table of the elements. It required the immense research and determination of the scientist Dmitri Mendeleev to show us that all elements followed a natural form and provided us with the first decent periodic table. There are 111 elements recognised today by IUPAC and they are arranged in the periodic table in horizontal groups and vertical periods.
The eight groups consist of the alkali metals, the alkaline earth metals, the transition elements, metalloids, non-metals, the halogens and the nobel gases. The development of the periodic table first began with Antoine Lavoisier. His job as a privatised tax-collector helped finance his scientific research. He was the first scientist to classified the elements into four groups. These groups consisted of gases, metals, non-metals and metal oxides. In 1789, he proposed the Law of Conservation of Mass. This law stated that the mass of the products of a chemical reaction is equal to the reactants.
This led to the “chemical revolution” and sparked interest amongst other scientists which, in turn, led to the periodic table that we know of today. In 1817 Johann Dobereiner saw became aware that the atomic weight of strontium was exactly half of the sum of the atomic weights of calcium and barium, which were elements that possessed similar properties. It took Dobereiner another twelve years to propose the Law of the Triads, after extensive research into finding the triads of the halogen group and the alkali metal group.
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In 1829 he proposed that nature contained triads of elements, with the middle element showing properties that were an average of the other two elements when ordered by atomic weight. Slowly, Dobereiners views began to be taken up by other chemists who tried to complete the unfinished triads, as further knowledge of the elements was gained. Dobereiner’s triads played an important role in Gmelin’s Hand Book of Chemistry, but besides their importance in this publication not much notice was taken of the triads until much later on.
The first scientist to arrange the elements in a periodic system was not actually a chemist, but a geologist. Beguyer de Chancourtois proposed a three-dimensional representation of the list of known elements wrapped around a cylinder in a helical graph. Elements that appeared on the same vertical line on the cylinder had similar properties. His helical graph also contained compounds and ions as well as elements so Beguyer de Chantcourtois’ work was disregarded until the work of Mendeleev.
In 1862, John Newlands wrote a paper in which he arranged the fifty-six known elements into eleven groups based on similar physical properties. He noted that many of the elements with similarities differed by some multiple of eight in their atomic weights. Newlands found his work unpublished by the Royal Society as there were many criticisms made about his classification of the elements. John Newlands left no places in the table for undiscovered elements which altered the flexibility of the scheme.
He didn’t evaluate the best values for the atomic weights, which was a serious omission according to Mendeleev. Some of the elements didn’t obey the scheme, the metals Mn, Ti and Fe aren’t of any resemblance to the non-metals P, Si and s which are placed eight elements before them. He was so convinced of his Law that he tried to force the elements to fit into this system Newlands believed that the system of the octaves would remain valid despite the number of elements that should be discovered. His work was ignored and forgotten until the work of Mendeleev had become famous.
Both Dmitri Mendeleev and Lothar Meyer produced similar results concerning the periodic table even though they worked independently of each other. Meyer constructed an abbreviated version of the periodic table, with only half if the known elements included. Meyer did not separate the elements of the sub-groups and main groups as Mendeleev did, but he did include the transition metals. Meyer had already predicted that there were undiscovered elements that would fit in his system, and so he left vacant spaces for them to be added to the table as they were discovered.
Meyer stated the Law of periodicity in 1868 which stated that ‘The properties of elements are largely periodic functions of their atomic weight, Identical or similar properties recur if the atomic weight is increased by a definite amount which is at first 16, then about 46, and finally 88 to 92 units’ Although Meyer’s table wasn’t used due to lack of certainty and flexibility, his colleague Seubert, from the University of Tubingen, republished his first papers in 1895, the year of Meyer’s death, so that students would be reminded of Meyers importance in the development of the periodic system.
Dmitri Mendeleev published his first periodic table in 1869. He arranged the elements in order of increasing atomic weight. Mendeleev created cards, with each elements symbol, atomic weight and its physical and chemical properties written on the cards. When these cards were arranged in order of increasing atomic weight then a table of the elements was formed which gave rise to the periodic table of the elements. There were gaps present in the table but Mendeleev didn’t see this as a problem. Instead he believed correctly that the gaps insinuated that elements were yet to be discovered .
From these gaps, he was able to predict accurately the physical and chemical properties of the undiscovered elements which he called eka-alumium, eka-boron and eka-silicon. The prefix eka- means similar to. He predicted that ten undiscovered elements existed and seven of these ten were discovered. Some say that his work and research was so brilliant because he wasn’t aware of the previous work done by Beguyer de Chancourtois, Dobereiner and Newlands. He didn’t accept the values for atomic weight without questioning the values.
If the element did not fit into the scheme, he simply changed the weight and re-positioned the element in a group where its chemical and physical properties were more suited. He also made some of the periods longer to accommodate what we now know today as the transition metals. Mendeleev’s table is arranged in rows and columns. The elements that we see today in a horizontal period were shown on his table in vertical columns and vertical groups were shown in horizontal rows. Below is an example of the periodic table of Dmitri Mendeleev.
Lord Rayleigh discovered a new inert, gaseous element in 1895 called argon. Although he knew of argon’s existence since 1983, it took him over a year to actually isolate the gas. Along with his colleague, William Ramsey, they noticed that this element didn’t fit into any of the known periodic groups. Ramsey made a suggestion that a new group should be formed and placed between chlorine and potassium in the periodic table. It was grouped with helium and a new family of elements was formed. Ramsey also correctly predicted the properties and identification of neon.
These inert, gaseous elements were labelled as the ‘zero’ group because of the zero valency of the elements. They were also called the inert gases for many years because they almost completely lacked in any chemical reactivity. It was only when Neil Bartlett in 1962 successfully made a compound which included the element xenon, that the group became known as the noble gases. It was found that xenon bound chemically to oxygen and fluorine and so it showed that there is indeed, limited reactivity in this group of elements. Between the ears of 1911 and 1914, Henry Moseley established the atomic numbers of the elements in the periodic table. This atomic number refers to the number of electrons in a neutral atom. He discovered the atomic number by using an experimental procedure which involved each element producing X-rays. He noticed that as the atomic weight increased, so did the energy of the X-rays. He didn’t understand this until he assigned numbers to the elements. This was a breakthrough in the development of the periodic table and gave rise to a periodic law.
This law states that ‘The properties of elements are periodic functions of their atomic number’. This Law was better than that of Mendeleev. When Moseley arranged the elements in order of increasing atomic number instead of increasing atomic mass then the irregularities that existed in Mendeleev’s table were gone. It is because of Moseley’s work that the periodic table that we have today is in existence. The final crucial change to occur in the period table of the elements came about from the work of Glenn Seaborg in 1940.
He discovered the transuranium elements 94-102, starting with plutonium. A year later, Seaborg and his team discovered the isotope plutonium-239. He found that this isotope could be used to construct a nuclear bomb due to the isotope being fissionable by bombardment with slow neutrons. The amount of the plutonium-239 was very little so he began working on how to increase the abundance of the isotope. This research led him to join the Manhattan Project to make bombs for the U. S. Army.
When the war ended he began focusing on the other trasuranium elements. Seaborg noticed that the heaviest elements were placed in the main body of the periodic table and he made another change to the table once again. He removed these heavy elements and placed displayed them separately from the main body of the table. He named the elements the Actinide series. As well as identifying the transuranium elements, Seaborg and his team brought to light more than 100 isotopes of the elements in the periodic table.
Seaborg received the honour of having an element named after him in appreciation of his extensive research into the periodic table. This element is named seaborgium (Sg). A transition metal is one which forms one or more stable ions which have incompletely filled d orbitals. Members of the transition elements and their compounds are good catalysts, probably due to their ability to change oxidation state. In the case of transition metals, they act as good catalysts because they are able to adsorb other substances onto their surface.
The 38 elements occur in groups three to twelve, and it is their valence electrons that occur in more than one shell that cause them to have many oxidation states. Most of the transition metals take a coloured form, as do some of their ionic compounds. Because of the electrons in the d subshell, they are separated into different energy levels, causing the elements to absorb the frequencies of white light. Hence they appear to be coloured. The discovery of new elements, largely due to research in radioactivity, has had an appreciable impact on the development of the periodic table.
The discovery of radioactivity in 1896 by Henri Becquerel inspired Marie and Pierre Curie to devote themselves to researching this area of chemistry. They succeeded in isolated radium and polonium just two years later. Marie measured the radiation given out by compounds of uranium and noticed that there was a similar radiation emitted by thorium compounds. During this time, she made the unexpected discovery that pitchblende contains a miniscule amount of an unknown radiating element. Pierre realised the importance of his wife’s work and joined in her research.
Over the next year they found two new elements and began work on isolating them so their chemical and physical properties could be established. The third radioactive element was found three years after the discovery of radioactivity when actinium was separated from pitchblende by Debierne. The discovery of these three elements and radon were not the only ‘new’ elements found. Approximately 35 more elements were discovered in the early twentieth century, but these were later identified as isotopes of elements that had already been discovered.
The identification of these isotopes and elements give rise to the modern periodic table of the elements. Some would say that although the periodic table is informative and a great discovery, the table does have some limits. Questions were raised in connection with the nonexistence of elements heavier than uranium. There were two main questions raised: Was it possible for heavier elements to occur in other parts of the universe? Was the lack of these elements caused by the instability of heavier atomic nuclei?
John Newlands was the only scientist associated with the periodic table to consider the possibilities beyond the limits of the atomic weights. He talked about its upper and lower limits and concluded that there was a simple association between the atomic weights and the ordinal numbers. Some scientists continued to leave vacancies ahead of hydrogen for the neutron, electron, alpha-particle or the hydrogen ion, even after it had been known that the order number was indicated by the number of protons in an atom of the element.
So today we are left with a period table of the elements that looks like this. There are eighteen groups and seven periods along with the lanthanoids and the actinoids in a separate grouping underneath the main body of the table. The table is laid out in such a way that the electron configuration for each element is valid as you go across a period. The periods get longer in the periods 4 – 7 to accommodate their electron configuration with s, p, d and f sub orbitals.
The periodic table that we have today is a valuable resource for means of education. It’s something that many people take for granted, not realising that it was built over centuries by many great scientists. The modern layout is easy to use and comprehend making the understanding of organic chemistry as a whole easier. Although there may be some doubts to the validity of the table, no one can deny the fact that this family tree of the elements is nothing short of a genius way to assist in the teaching and learning of chemistry.
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