THE DEVELOPMENT OF THE PERIODIC TABLE
At the start of the 19th century, chemists noted that while the known elements had different properties, some of these had distinct similarities. One such chemist, Johann Wolfgang Dobereiner, grouped the elements into three which he called "triads." For example, lithium, sodium, and potassium belong to one triad. Other triads included calcium, strontium, barium; chlorine, bromine, and iodine.
In 1865, John A. Newlands proposed a new classification of elements. At that time, there were about 62 elements to sort. Newlands found that if the elements were arranged according to increasing atomic weight, chemical properties repeated in an interval of eight. The first element behaves in a similar way as the eight element. Newlands then declared this as the law of octaves. Two examples are: Li, Be, B, C, N, O, F. Ne; Na, Mg, Al, Si, P, S, Cl, and Ar.
In 1869, Dmitri Mendeleev (1834-1907) in Russia, and Lothar Meyer in Germany, independently proposed an almost identical way of arranging the elements. Their arrangement of the elements was according to increasing atomic weights, no different from Newlands'. However, Mendeleev's arrangement was distinguished for several reasons. First, he was the first to tabulated the elements. In his arrangement, elements that fell in the same column showed similar chemical and physical properties. He also used cards containing information on each element, similar to the boxes in today's periodic table. Most importantly, Mendeleev left vacant spaces or breaks for elements that he inferred had not been discovered yet at that time. He went on to predict the properties of three yet-to-be-discovered elements. Shortly after his periodic table came out, two of the elements he predicted were discovered (gallium and scandium). Followed some years later by the discovery of the third (germanium). In all instances, there was a striking resemblance between the predicted properties made by Mendeleev and the properties of the newly discovered element.
The success if Mendeleev's prediction stimulated other chemists to adopt his table, which was based on the periodic law that was modified by Moseley later. Mendeleev's periodic law is stated thus: when the elements are arranged according to increasing atomic weight, certain sets of properties repeat periodically. Mendeleev noted that if he would adhere to this law strictly, some of the elements seemed misplaced. So, he simply switched them and placed them with the group which ad the same properties. He thought that this error was to to inaccurate atomic weights.
Through experiments which used x-rays. Henry Moseley discovered in 1913 a functional relationship that allowed him to use spectral lines to determine the number of protons, and therefore, the atomic number of the elements. Moseley saw that the errors in Mendeleev's table could be corrected if, instead of arranging according to atomic weights or atomic masses, atomic number should be used. So, he used the atomic numbers to predict three more yet-to-be-discovered elements, namely: technetium, promethium, and rhenium. These were discovered in 1937, 1945, and 1925, respectively. On the basis of these developments, the periodic law can be stated as follows:
"When the elements are arranged according to increasing atomic numbers, certain properties repeat periodically."
Today, there are more than 100 known elements, arranged significantly in increasing atomic number in a table with seven rows and eighteen columns. The place of an element in these columns and rows reveals its properties in relation to the rest of the elements around it.
If we observe the names of the elements in the periodic tables, we may notice that their names were derived from any of the following:
- Latin Names - Examples: natrium (Na) for sodium, ferrum (Fe) for iron, and Kalium (K) for potassium.
- Properties of the Element - Examples: iodine, I, (from the word iodes meaning violet); fluorine, F, (from the word fluere meaning flow); and tungsten, W, (from a Swedish word meaning hard).
- Mythological figures - Examples: niobium, Nb, (from Niobe, daughter of King Tantalus); tantalum, Ta, (from King Tantalus); and thorium, Th, (from the Norse god Thor).
- Name of the Discoverer or Scientists - Examples: einsteinium, Es, (Albert Einstein); curium, Cm (Pierre and Marie Curie); and nobelium, No, (A. Nobel, the discoverer of dynamite).
- Place/Country of Discovery - Examples: francium, Fr, (France); Germanium, Ge, (Germany) and Californium, Cf, (California, USA).
- Atomic Number - Elements of atomic number 104 and higher are assigned names based on atomic number. Examples: unnilquadium, Unq (from un - 4 nil - quad - ium or 1-0-4); unnilhexium, Unh, (atomic number 106); and unnilseptium, Uns, (atomic number 107).