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               Genesis of Periodic Classification and Mendeleev Periodic Table:  
                
               Classification of elements and periodicity in properties: 
               Periodic  table  helps  in  the  systematic  study  of  most  of  the  elements  found  in  nature.  The  elements  are 
               classified into different groups and periods in the periodic table which helps to study about the compounds 
               formed by those elements. Moreover the analysis of the ionization energy, electronegativity, electron affinity 
               etc is also possible. 
                
               Dobereiner triad rule: 
               Dobereiner pointed out that the atomic weight of an element present in the middle in a group of three 
               elements is equal to the mean of the first and third element only if they possess the same physical and 
               chemical property. 
                Representation of Dobereiner triads: 
                
               Triad elements                                Li                   Na               K       Ca                       Sr         Ba 
               Atomic weight                                 7                   23               39       40                      88         137                                      
               Mean value                                                      23                                                88.5 
                
               Drawback: His rule was unable to arrange all the known elements in a triad. 
                
               Law of Octaves: Newlands founded this law in 1865 in which the elements were arranged in order of 
               increasing atomic weights. Every eighth element was found to possess similar properties to that of the first 
               element. This law was given this name because of its resemblance with the octaves of music where the eighth 
               note resembles the first. 
               Drawback: His law seemed to be correct only for elements up to Calcium. 
                
               Mendeleev’s periodic table: 
               Mendeleev published the periodic law for the first time which states as ‘the properties of the elements are a 
               periodic function of their atomic weights.’ 
               Characteristics: 
               i)it is based on the atomic weight. 
               ii)63 elements were known and noble gas were not discovered. 
               iiiThe elements were arranged in a horizontal row called periods and vertical column as groups in the periodic 
               table in order of increasing atomic weights. 
               iv) There were 7 periods and 8 groups in Mendeleev’s periodic table. 
               v)Each group upto 7th is divided into two subgroups A and B. A subgroups are normal elements and B 
               subgroups are termed as transition elements. 
               vi)The 8th group consist of 9 elements in three rows. 
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                    vii)the properties of elements belonging to same grop possess same properties. 
                     
                    Merits: 
                    i)Study  of  the  properties  of  the  elements  became  easier  when  the  elements  were  classified  in  groups 
                    according to their properties. 
                    ii)His law encouraged for the prediction of new elements. 
                    iii)Correction were done in atomic weights of some elements. 
                    Atomic weight= valency x equivalent weight. 
                    Demerits: 
                    i)uncertainty in the position of hydrogen.lear. 
                    ii)isotopes were not given separate positions. 
                    iii)lanthanides and actinides were related to IIA or IIB was not clear. 
                    iv) the order of increasing atomic weights were not followed strictly. 
                     
                    Modern Periodic Law and present form of Periodic Table : 
                     
                    Modern periodic table: 
                           I)            Moseley proposed the Modern periodic table. 
                           II)           Atomic number is the basis of this law. 
                           III)          A high speed electron was bombarded on different metal surface to obtain X-rays.It was found out 
                                         that        v is proportional to Z where v is the frequency of X-ray and 
                                         Z = atomic number. 
                                          
                    Modern periodic law:  This law is based on the assumption that the physical and chemical properties of 
                    elements are the periodic function of their atomic number. 
                    Characteristics: 
                           i)            The 9 vertical columns present were called groups 
                           ii)           1st to VIIIth group+ group of inert gases. 
                           iii)          Ramsay introduced the inert gases . 
                           iv)           The 7 horizontal series termed as periods. 
                           Long form of periodic table: 
                           i)            Based on concept of  Bohr Bury electronic configuration and atomic number. 
                           ii)           Rang and Werner proposed this model. 
                           iii)          7 periods and 18 columns. 
                           iv)           The 18 vertical columns are named as 1st to 18th group according to IUPAC 
                           v)            The outermost shell of the elements present in the same group possess equal number of 
                                         electrons. 
                           Li, Be, B ,the second period elements shows diagonal relationship with Mg,Al,Si 
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                of third period elements . 
                Na,Mg,Al,Si,P,S,Cl of the 3rd period represents properties of other elements in the respective group. 
                Atomic number of the last inert gas is 86. 
                Number of gaseous elements present =11 
                Number of liquid elements=6 
                Number of of solid elements =95 
                     nd
                The 2  period possess maximum number of gaseous elements (N,O,F,Ne respectively) 
            Electronic Configuration of Elements  
             
                s block elements: 
                   1.The elements in the periodic table  whose last electron enters the s orbital are termed as the s block 
                    elements. 
                   A  maximum of two electrons can be accommodated in the s orbital. 
                   ns1 and ns2 are the general formula of the elements of this block. n varies from 1 to 7. 
                    The first group elements are known as alkali metals as the reaction with water leads to the formation 
                    of alkali.the second group elements are known as the alkaline earth metal because of the reaction of 
                    their oxides with water to form alkali. 
                   14 elements are present in the s block. 
                    H and He are gaseous elements whereas Fr87 and Ra88 are radioactive. 
                   liquid elements belonging to s block are Cs and Fr. 
                p block elements: 
                   The elements in the periodic table whose last electron enters the p orbital are termed as the p block 
                    elements 
                   A maximum of six electrons can be incorporated in the p-orbital. 
                   General formula for the p;block elements is ns2p1-6. Where n=2 to 6. 
                   The elements having the formula ns2p6 are inert as their energy level is filled. 
                   Excluding he , a total of 30 elements are present in the p block 
                   Ga and Br are liquids  
                   The step like structures in the periodic table seperates the elements into  metals, non metals and 
                    metalloids. 
                d block elements: 
                   The elements in the periodic table whose last electron enters the d orbital are termed as the d block 
                    elements. 
                   The electrons gets filled up in the penultimate shell in the d orbital 
                   Between s and p block elements, the d block elements lie. 
                   The general formula of these elements  is ns2p6d1-10 where n=4 to 7. 
                   The elements present in this block are metals. 
                   Mercury is the only liquid element present in d block. 
                     
                    f block elements: 
                       The elements in the periodic table whose last electron enters the f orbital are termed as the f 
                        block elements 
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