Periodic Table in Chemistry
The table in which the elements are arranged in the increasing order of their atomic number is called periodic table. The similar properties which recur after a regular interval are called the periodic properties. Several attempts were made to classify the elements so as to make their study simple and systematic. J.W. Dobereiner arranged the elements in the group of three called triads. According to this, the chemically similar elements were arranged in the increasing order of their atomic weights in the group of three to make a triad. It was noted that the atomic weight of middle element present in a triad is the mean of the other two elements.
Triad of alkali metals – lithium, sodium and potassium have atomic weights 7, 23 and 39 respectively. By taking the arithmetic mean of lithium and potassium we get the atomic weight of sodium as 7+39/2 = 46/2 = 23.
Triad of halogens – chlorine, bromine and iodine have atomic weights 35.5, 80.4 and 126.5 respectively. By taking the arithmetic mean of chlorine and iodine, we get the atomic weight of bromine as 35.5 + 126.5/2 = 81
Triad of alkaline earth metals – calcium, strontium and barium have atomic weights 40, 88 and 137 respectively. By taking the arithmetic mean of calcium and barium, we get the atomic weight of strontium as 40+ 137/2 = 88.
This classification was not accepted as many elements could not be grouped in triads. For very low mass or for very high mass elements, the law was not holding good, eg. F, Cland Br. Atomic mass of Cl is not the arithmetic mean of atomic masses of F and Br.
In periodic table the vertical lines are called groups and the horizontal lines are called periods. Mendeleev suggested the periodic law, ie., the physical and the chemical properties of elements are the periodic functions of their atomic weights.
Mendeleev’s periodic table had certain defects:
Position of hydrogen: Hydrogen had a controversial position, i.e. it was placed in two groups, group 1 (alkali metals) and group 17 (halogens).
When Mendeleev presented his periodic table inert gases were not discovered, he easily placed these elements at the end of the table without disturbing the table.
The elements which were discovered later could be easily placed in the gaps provided in the table.
The most accepted form of periodic table is the long form of the periodic table. In 1913 Mosley modified the previous periodic law as it states that the physical and the chemical properties of the elements are the periodic functions of their atomic number. As Mosley found that the atomic number (i.e. the number of protons) is more fundamental property of an atom than its atomic weight.
On the basis of the modern periodic law proposed by Mosley, the long form of Modern or Bohr’s periodic table was prepared, which consists of 18 vertical lines (groups) and 7 horizontal lines (periods). In the long form of periodic table the elements having same number of valence electrons are placed under the same vertical columns called groups.
The elements present in different groups are given specific names like
|2||Alkaline earth metals|
|18||Inert Gases or Noble Gases|
The horizontal rows are called periods.
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