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 "PERIODIC CLASSIFICATION".
A group of three elements, which have similar physical and chemical properties, is known as "TRIADS".
  In 1829, a German scientist Dobereiner made use of the relationship between atomic masses and properties of   elements. He proposed,
  "If three any 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
Li (7), Na (23), K (39)
atomic mass of Na
Ca (40), Sr (87), Ba (137)
nearly equal to the atomic mass of Sr
S (32), Sc (79), Te (128)
nearly equal to the atomic mass of Sc
Cl(35.5), Br(80), I(126.5)
35.5+126.5 / 2 = 81
nearly equal to the atomic mass of Br
  Drawback:This rule is not for all the elements.
  In 1866, a British scientist, Newlands, reported his "law of octave" by arranging elements according to   increasing order of their atomic masses. He noticed that "Every eight element, starting form any point,   approximately has similar properties".
  Newlandís arrangement was applicable only to a few elements after that it was failed.

  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

  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
  In 1864, a German Chemist Luther 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.

Figure (curves)


  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 because,   according to him fundamental property of an element was atomic mass. He arranged these eight   groups were further divided into subgroups.
  Mendeleevís periodic table was very precise and provided the basis of modern periodic classification.
  Mendeleevís periodic table offered the following advantages in understanding the properties of   elements.
  1. There was a regular gradation in physical and chemical properties of elements.
  2. The group number of an element indicates highest oxidation state that it can attain.
  3. There were many vacant spaces in Mendeleev's periodic table for the elements to be discovered. He      named them Eka-Boron, Eka-Aluminium and Eka-silicone 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.
Original statement of Mendeleev's periodic law
    Published: March 1869
  1. The elements, if arranged according to their atomic weights show an evident periodicity of       properties.
  2. Elements which are similar as regards their chemical properties have atomic weights which are either       nearly the same value or which increase regularly.
  3. The arrangement of the elements or of groups of elements in the order of their atomic weights,       corresponds with their so called valencies.
  4. The elements which are most widely distributed in nature have small atomic weights, and sharply       defined properties. They are therefore typical elements.
  5. The magnitude of the atomic weight determines the character of an element.
  6. The discovery of many as yet unknown elements may be expected.
  7. The atomic weight of an element may sometimes be corrected by the aid of a knowledge of those of       adjacent elements.
  8. Certain characteristic properties of the elements can be foretold from their atomic weights.
  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 a clear idea about the structure of atom.
  3. Lanthanide and Actinide have been assigned places 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 is no separate position for isotopes in his periodic table.
  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.