Class- X 
                    Chemistry 
                    Chapter-5 
               Periodic Classification of Elements 
      ‘’The periodic table is a tabular method of displaying the elements in such a 
      way, that the elements having similar properties occur in the same vertical 
      column or group”. 
      Without the classification of elements, it would be extremely difficult and 
      time-consuming to individually study the chemistry of all the elements.  Hence, 
      to simplify and systematize the study of elements and their compounds, they 
      are classified into groups and periods. 
      Early Models of Periodic Table 
      Dobereiner’s Triads 
      Dobereiner arranged a group of three elements with similar properties in the 
      order of increasing atomic masses and called it a triad. He showed that the 
      atomic mass of the middle element is approximately the arithmetic mean of 
      the other two. But, Dobereiner could identify only following three triads from 
      the elements known at that time. 
                                       
      Drawbacks of Dobereiner’s Triads 
      Dobereiner could identify only three triads from the elements known at that 
      time. Hence, this system of classification into triads was not found to be useful. 
              Newlands’ Law of Octaves 
              According to this ‘when elements are placed in order of increasing atomic 
              masses, the physical and chemical properties of every 8th element are a 
              repetition of the properties of the first element.’ 
              Newlands compared these octaves to the series of eight notes of a musical 
              scale. 
                sa (do)    re (re)     ga (mi)        ma (fa)       pa (so)        da (la)       ni (ti) 
                H          Li          Be            B             C               N              O 
                F          Na          Mg            Al            Si              P              F 
                Cl         K           Ca            Cr            Ti             Mn             Fe 
                Co and  Cu             Zn            Y              In             As            Se 
                Ni 
                Br         Rb          Sr            Ce and La     Zr              ______        _____ 
              Limitations of Newlands’ Law of Octaves 
                  •  Law of octaves was applicable only up to calcium (only for lighter 
                     elements). As after calcium every eighth element did not possess 
                     properties similar to that of the first. 
                  •  Newland adjusted two elements in the same slot (e.g., Co and Ni), 
                     having different properties. For example; Co and Ni with Fluorine, 
                     Chlorine, Bromine and Iodine. 
                  •  In order to fit elements into his Table, Newlands adjusted two elements 
                     in the same slot, but also put some unlike elements under the same 
                     note. For example, cobalt and nickel are in the same slot and these are 
                     placed in the same column as fluorine, chlorine and bromine which have 
                     very different properties than these elements. Iron, which resembles 
                     cobalt and nickel in properties, has been placed far away from these 
                     elements. 
       Mendeleev’s Periodic Table:  
       Mendeleev’s periodic table is based on the physical and chemical properties of 
       elements and their atomic masses. 
       Mendeleev’s Periodic Law 
       According to this “The physical and chemical properties of the elements are 
       the periodic function of their atomic masses.” 
        
                                                     
       Periodicity of Properties:  
       The repetition of properties of elements after certain regular intervals is 
       known as Periodicity of Properties. 
       Features of Mendeleev’s Periodic Table 
       ● Six horizontal rows, known as periods. 
       ● Eight vertical columns known as groups. 
       ● Groups I to VII subdivided into A and B subgroups. 
       ● Groups VIII doesn’t have any subgroups and contains three elements in each 
      row. 
      ● Elements in the same group exhibit similar properties. 
      Achievements of Mendeleev’s Periodic Table 
      A systematic study of elements: 
       Elements with similar properties were grouped together, that made the study 
      of their chemical and physical properties easier. 
      Correction of atomic masses:  
      Placement of elements in Mendeleev’s periodic table helped in correcting the 
      atomic masses of certain elements. For example, the atomic mass of beryllium 
      was corrected from 13.5 to 9. Similarly, atomic masses of indium, gold, 
      platinum etc., were also corrected. 
       Prediction of properties of yet to be discovered elements:  
      Mendeleev’s left vacant places in his table which provided an idea for the 
      discovery of new elements. Example: Eka-boron, Eka-aluminium and Eka-
      silicon. 
       Mendeleev’s periodic table was predicted properties of several         
      undiscovered elements on the basis of their position in Mendeleev’s periodic 
      table. 
       These elements, when discovered were named scandium, gallium, and 
      germanium respectively.