PERIODIC CLASSIFICATION
 
Introduction
   When a very large number of elements become known to scientists , it was felt that they must be    arranged in a systematic order .Because a systematic classification provides a clear idea and    information about the properties of elements and make it easier to predict the properties of different    elements. This classification provides the inter-relationship of scientific facts.
   In old days, element were arranged in the ascending order of their atomic masses.
   The orderly arrangement of elements is called "PERIOD CLASSIFICATION".
DOBEREINERíS TRIADS
   A group of three elements, which have similar physical and chemical properties, is called    "TRIADS".
   In 1829, a German scientist made use of the relationship between atomic masses and the properties
   of elements. He proposed,
   "If three elements are arrange in ascending order of their atomic masses, such that the
    atomic mass of middle element is Arithmetic mean of the first and third elements, then these    element will show similar properties".
   This is known as "Law of Triads". This rule is applicable only in a few cases.
   For example
TRIADS
ARITHMETIC MEAN
Result
   Li (7), Na (23), K (39)
7+39/2=23
atomic mass of Na
   Ca (40), Sr (87), Ba (137)
40+137/2=88
nearly equal to the
atomic mass of Sr
   S (32), SC (79), Te (128)
32+128/2=80
nearly equal to the
atomic mass of Sc
   Drawback:This rule is not valid for all the elements.
NEWLANDíS LAW OF OCTAVES
   In 1866, a British scientist, Newland, reported his "law of octave" by arranging elements according to    increasing order of their atomic masses. He notified that "Every eight element, starting form any point,    approximately has similar properties".
   Newlandís arrangement was applicable only a few elements after that it was failed.
   ADVANTAGES OF THE LAW

   1. This law provided a basis for the classification of element into groups of elements having similar    properties.

   2. This law provided a wider scope to arrange all known elements into a tabular form

   DISADVANTAGES OF THE LAW
   1. Newlandís law is not applicable to all the elements.
   2. This arrangement did not include NOBLE GASES because they were not discovered then.
   3. Heavier elements could not be accommodated
LOTHER MEYERíS CLASSIFICATION
   In 1864, a German Chemist Lothar Meyer published on incomplete periodic table .He includes about 56    elements arranged in a group from I to VIII. He plotted the values of different physical properties and    obtained different curves .In these graphs, he observed that element with similar physical properties    occupy similar positions in the curve
   For example
   Alkali metals occupy the peak of the curves.

   Halogen occur on the ascending portions of the curve.

MENDELEEVíS PERIODIC TABLE

   In 1869, a Russian chemist, Mendeleev, on the basis of physical and chemical properties discovered a    relation known as "PERIODIC LAW".
   Mendeleevís Periodic Law
   According to the law:
"The properties of element are the periodic function of their atomic masses".
   Mendeleev arranged the known elements according to increasing order of their atomic masses.    According to him, fundamental property of an element was atomic mass. He arranged these elements
    in eight groups. They were further divided into sub-groups.
   Mendeleevís periodic table was very precise and provided the basis of modern periodic classification.
   ADVANTAGES OF MENDELEEVíS PERIODIC TABLE:
   Mendeleevís periodic table offered the following advantages in understanding the properties of    elements.
   1. There was a regular gradation in the physical and chemical properties of element.
   2. The group number of an element indicates highest oxidation state that it can attain.
   3. There were many vacant spaces in table for the elements to be discovered. According to his    prediction, he named them Eka-Boron, Eka-Aluminium and Eka-silikon. He also predicted the properties    of these undiscovered elements including atomic masses. These elements were discovered as Sc ,Ga    and Ge with same features as he predicted.
   4 .Mendeleevís arrangement helped to correct atomic masses of a number of elements.
   DISCREPANCIES IN MENDELEEVíS PERIODIC TABLE

   1- For placing the elements in proper groups, the order of the elements according to atomic mass was    reversed in certain cases. He placed Iodine (127) after Tellurium (128) Potassium (39) and Ni (58)
   after Co (59). Which is against his periodic law but correct according to properties.

   2- Mendeleevís periodic table does not provide clear idea about the structure of atom.

   3- Lanthanide and Actinide have been assigned placed in the periodic table which is against the    periodic law.

   4- Alkali metal and coinage metals (Cu, Ag and Au) which differ widely in properties are placed into the    same group.

   5. There was no separate position for isotopes in his periodic table.
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   6. The change in atomic mass of two successive elements is not constant. Hence it is not possible to    predict the number of missing elements by knowing the atomic masses of two known elements.

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