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.

Brief History of development of Periodic Table.

Lavoisier classification of elements

In the beginning of 19th century Lavoisier classified elements to Metals and Non metals.

But this classification is not enough to study large number of elements in easy way.

So, to make it more simpler there was several attempts .

Dobereiner's triads

                              Johann_Wolfgang Dobereiner

In 1829 German chemist noticed that there is a relationship between atomic weight and properties of elements. 

He make a group of three elements having similar properties, he arranged them in such a manner that the arithmetic mean of mass of first and third element is equal to the mass of middle one.

                            Examples of some triads 

Achievements of Dobereiner triads

Dobereiner achieved to build a relationship between properties of  elements and atomic mass.

Limitations of Doberieners triads

At the time of doberiener only 30 elements were known but he was unable to fit all the elements into triads. He could able to make just three triad's.

Newland's Octaves

In 1863 British Chemist John Newland proposed a new system of grouping of elements.

According to him, If elements were arranged according to increasing atomic weight then property of every eight element is similar to the first element just like a musical note.

For example

Achievement of NewLand's Octaves

This is a better way to grouping elements. In this arrangement triads of doberiener can be seen. 

In this way more number of elements can be classifieds into groups.

Limitations of Newlands Octave.

• Newland arranged all 56 known elements in his classification but his law of octave is valid up to calcium (mass=40) only.
• New discovered elements (having small mass) cannot be placed in his classification.
• He just make a sequence of elements, he didn't make any table or boxes.

Lother Mayer's Arrangement

In 1869 a German chemist Lother Mayer plotted a graph between atomic mass and atomic volume. He noticed that elements having similar properties occupies the same position in his graph. This arrangement is accurate and matched today's modern periodic table. But still it was lacking.

Lacking of this arrangement

It's too tough to remember and explain the positions of elements. 

Mendeleev's periodic Table

In 1869 a Russian scientist D.I Mendeleev classified elements in a systematic manner.

He arranged elements according to his law.

Mendeleev's periodic law

"Chemical and physical properties of elements are periodic functions of their atomic masses."

Mendeleev arranged all the known 63 elements in the form of table. This classification has vertical coloums called Group and horizontal rows called periods.

Structure of Mendeleev's periodic table

Mendeleev kept elements in boxes, according to increase in atomic mass. In the top of group he has written the general formula of their oxides and hydrides.

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Importance of classification of elements

At the last of 17th century only a few elements were known, but in 18th and 19th century elements were discovered frequently, with discovery of new elements , study of elements becomes difficult.

To make study easier there is a need of classification of elements.

Today it's very easy to study all the physical and chemical properties of 118 known elements, this is possible because  of well classification of elements known as "modern periodic table" or "long form of the periodic table".

But this periodic table is not designed at once, there is a great history of classification of elements.

Also read:- brief history of classification of elements