Philosophers of chemistry consider the chemical elements in two distinct ways:
Firstly, there is the chemical element as basic substance, that is: the element as a bearer of properties but not having any actual properties, except for atomic mass and atomic number Z. Chemical symbols, such as H and Cu, are assigned to the element as basic substance.
Secondly, there is the element as simple substance, for example, a piece of copper metal placed on a table has numerous, measurable, intrinsic properties such as: purity, density, conductivity, colour, melting point, molar volume, etc. |
Crucially, only the element the basic substance survives in a compound. Sodium's metallic properties and 'chlorine, the green gas' do not exist in the ionic salt, sodium chloride.
The gaseous elements, nitrogen, hydrogen, oxygen, and chlorine, had all been discovered in the eighteenth century. So had the metals, cobalt, platinum, nickel, manganese, tungsten, molybdenum, uranium, titanium, and chromium. In the early days, the elements were classified into metals and nonmetals.
Today 116 elements are known , out of which upto 97 occur in nature and rest are synthesized in laboratories. The elements varied widely in properties and there seemed little order about them. Why were there so many→ And how many more yet remained to be found? Ten? Fifty? A hundred? A thousand? An infinite number? |
Scientists knew that that certain properties of elements occur periodically when arranged by some order (although they were not sure which parameter to be used). These similarities can be reflected best by a table, so that commonalties between elements appear both in rows and in columns of the table.
Different scientists attempted to find some order in the list of elements already known. For example, Mendeleev arranged the elements according to their increasing atomic weights (although this led to some inconsistencies). Perhaps in this manner some reason for the number of elements might be found and some way of accounting for the variation of properties that existed. Some of the prominent attempts are described further. |
Johann Dobereiner
(1780 - 1849) | Dobereiner's law of triads :
In 1829, german chemist Johann Wolfgang noted that the element bromine, seemed just halfway in its properties between chlorine and iodine. Not only did chlorine, bromine, and iodine show a smooth gradation in such properties as color and reactivity, but the atomic weight of bromine seemed to lie just midway between those of chlorine and iodine. Coincidence? |
Dobereiner went on to find two other groups of three elements exhibiting neat gradations of properties: calcium, strontium, and barium; and sulfur, selenium, and tellurium. In both groups the atomic weight of the element in the middle was about midway between those of the other two. Coincidence again ? Dobereiner called these groups "triads", and searched unsuccessfully for others.
The fact that five-sixths of the known elements could not be fitted into any triad arrangement made chemists decide that Dobereiner's findings were merely a coincidence. |
John Newlands
(1837-98) | Newlands' law of octaves :
In 1864, the English chemist John Alexander Reina (1837-1898) arranged the known elements in order of increasing atomic weights, and noted that this arrangement also placed the properties of the elements into at least a partial order. When he arranged his elements into vertical columns of seven, similar elements tended to fall into the same horizontal rows. Thus, potassium fell next to the very similar sodium; selenium fell in the same row as the similar sulfur; calcium next to the similar magnesium, and so on. Indeed, each of Dobereiner's three triads were to be found among the rows. |
Newlands called this the law of octaves (there are seven notes to an octave in music, the eighth note being almost a duplicate of the first note and beginning a new octave.) In other words Newlands found that when the elements were arranged in the order of their increasing atomic weights, every eighth element had properties similar to those of the first one. Unfortunately, while some of the rows in his table did contain similar elements, other rows contained widely dissimilar elements. It was felt by other chemists that what Newlands was demonstrating was coincidence rather than something of significance. He failed to get his work published. |
Dmitri Mendeleyev
(1834 -1907) | Mendeleev's periodic law :
Mendeleyev is called the father of the modern periodic table. Mendeleev avoided Newlands's mistake of insisting on equal periods throughout. He had also discovered the change in length of the periods of elements. Mendeleev tackled matters from the direction of valence. Mendeleev insisted that his periodic classification system concerned the elements as basic substances possessing only one attribute, atomic weight. Therefore he arranged the elements according to their increasing atomic weights and showed that certain characteristic properties of the elements reappear at definite interval and they are periodic functions of their atomic weights. This is called Mendeleyev's periodic law. |
Mendeleyev's periodic table contains 8 vertical columns of elements called groups and 7 horizontal rows called periods. Each group from Group I to Group VII, has two sub-groups A and B The properties of the elements in the same sub-group are similar.
The important features/merits of Mendeleyevs periodic table are:
(1) The table gave for the first time a systematic summary of the available information of the elements and their compounds.
(2) The elements are arranged in such a way that those belonging to one sub-group show similar chemical properties.
(3) Considering the positions of certain elements in the periodic table, their doubtful atomic weights were corrected e.g., the atomic weight of berylhum was corrected from 14. 9 to 9.
(4) Mendeleyev had left three vacant places in the table for elements not known at that time. He called them eka-boron, eka-aluminium and eka-silicon. These elements were discovered subsequently and were named scandium gallium and germanium, respectively. Mendeleyev had already predicted the properties of these elements based on their positions in the periodic table. These were found to be correct.
(5) The table could be used to predict approximately the atomic weights and the properties of any new element.
In order to make the elements fit the requirements that those in a particular column all have the same valence, Mendeleev was forced in one or two cases to put an element of slightly higher atomic weight ahead of one of slightly lower atomic weight. Thus, tellurium (atomic weight 127.6, valence 2) had to be put ahead of iodine (atomic weight 126.9, valence 1) in order to keep tellurium in the valence-2 column and iodine in the valence-1 column.
The demerits of Mendeleyevs periodic table are as follows:
(1) The elements having similar chemical properties are not necessarily in the same group. For example, mercury, lead and barium are placed in different groups although they posses similar properties.
(2) On the other hand, some elements having different chemical properties are placed in the same group. For example, copper, silver and gold are placed in the group of alkali metals. Actually these metals exhibit different properties than those of the alkali metals.
(3) Some elements with higher atomic weights are placed before those having lower atomic weights. For example, argon with atomic weight 39. 9 is placed before potassium having atomic weight 39.1.
(4) Isotopes cannot be placed separately in this table. |
|
0 comments:
Post a Comment