Brief study of Mendeleev's periodic table.

 In 1869, A Russian scientist D.I Mendeleev has done a well classification of elements. He not only studied the properties of elements, also studied the properties of their compound and given a law which is called Mendeleev's periodic law

"Physical and chemical properties of elements are the periodic function of their atomic mass"

He has drawn a table with increasing mass of elements, which is called mendeleev's periodic table. Which contains groups and periods.

He called the vertical columns having similar properties as groups and horizontal rows as periods.

Characteristics of Mendeleev's periodic table

• In modern Mendeleev's table it has 9 columns, which called groups, and these groups are called group Ⅰ, Ⅱ, Ⅲ, Ⅳ, Ⅴ, Ⅵ, Ⅶ, Ⅷ, ０ respectively.
• First 7 Groups are again divided into sub-groups of "A" & "B". 
• Group Ⅷ contains total 9 elements, which is kept in the form of traids.
• In 0th group all nobel gases are placed.

It has 7 horizontal row called periods.

              

Achievements of Mendeleev's Periodic table

1. Makes study of elements easier:- It was the first classification of elements in the form of table, which was very systematically. In this classification elements were arranged in groups, so properties of several elements can be studied by study of the group.
2. Helpful for prediction of reacting behavior of elements:- with the help of mendeleev's periodic table we can guess how's elements react with each other
3. Helpful for prediction of undiscovered elements:- At the time of Mendeleev's only 56 elements were known but he left gaps for undiscovered elements, which was discovered later, he named them eka- boron, eka-aluminium and eka-silicon, which is known today as scandium, gallium and germanium respectively.
4. Helps in mass correction of elements:- Mass of some elements were wrongly used at that time, which was corrected by Mendeleev. For example-- mass of beryllium was considered as 13.5 at that time because it's vacancy was taken as 3 during mass calculation. Formula for mass calculation 

          Mass = equivalent weight × vacancy

                     = 4.5 × 3

                     =13.5

         Mendeleev corrected the valancy of  beryllium to 2

        Mass = equivalent weight × vacancy

                     = 4.5 × 2

                     =9

       Hence, mass of beryllium changed from 13.5 to 9

Though  it has several achievements but still need corrections because of some demerits.

Demerits of  Mendeleev's periodic table

1. Position of Hydrogen:- Hydrogen is the only element which resemble Ⅰ(A) group elements as well as Ⅶ(A) group elements ,so the position of hydrogen is not fix.
2. Position of isotopes in periodic table:-  Mendeleev's classification is based on the mass of elements and isotopes of elements were also present in nature. If elements were arranged on the basis of mass then  the isotopes should be placed in the periodic table.
3. Placing of similar elements in different group:- silver and thallium, barium and lead, copper and mercury shows similar properties but they paced in different groups.
4. Placing of heavier element before lighter elements:- According to Mendeleev's law , elements should be arranged according to increasing atomic mass but he kept some heavier elements before lighter elements which violent his own law. For example- Cobalt (58.93) placed before Ni(58.69) ,Argon(39.9) placed before Potassium (39.1)
5. Placing of different elements in same group:- Although a group is divided into two subgroups, there are considerable disparities between the elements placed in these two subgroups "A" and "B". For example Ⅰ(A) contains Li, Na, K etc, which are soft metals whereas Ⅰ(B) contains Cu, Ag, Au which are hard metals and used for making of coins.
6. No explanation of periodicity:- Mendeleev did not explain the reason for the periodicity among the elements.

For above mentioned limitations of Mendeleev's periodic table, Scientists began to realize that mass is not a periodic function of elements, instead of mass there must be something else.