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periodic classification of elements class 10 | class 10 science chapter 5 notes

periodic classification of elements class 10 | class 10 science chapter 5 notes

         History of periodic table


A periodic table is a table that gives info about all the elements which have been found till  now, including gas , metals & non-metals, metalloids etc..Till  now 118 elements are found and out of which only 98 is naturally occurred .

  

 

   Now the question comes that  who does this table is being classified ?... Lets discuss briefly about it. in the given section below:


    Dobereiner Traids:

 The first attempt for the classification of elements was done by Johann Wolfgang Dobereiner (a German Chemist).In his attempt he tried to arrange some elements with similar properties into  some groups. He identified some groups having three elements each and he named these groups as  Traids. 

  

 dobereiner triads definition:


dobereiner triads elements

  Dobereiner's Traids states that "when three elements in a traids were written in order of increasing atomic masses,the atomic mass of the middle element was roughly the average of the atomic mass of the other two elements".



dobereiner triads limitations: The Drawback of Dobereiner traids is that he could identify only three traids from the elements known at that time. Later this law wasn't  useful for classification of  elements.

 

  Newlands law of octaves:


After the failure of Dobereiner traids,  an English scientist  John Newlands in 1886 tried an attempt to classify elements.In his classification he arranged the known elements in increasing atomic masses.He started his classification from hydrogen(i.e. lowest atomic mass element) and ended his classification to thorium which was the 56th element.

 Newland's law of octaves states that the eighth element had properties similar to that of the first.He named this eight elements as Octaves because he  compared this to the octaves found in  music.

 

Newlands law of octaves example:

Newlands law of octaves table        

Lets us take an example of Lithium and  sodium  from the above table.By applying this law it shows that the properties of Lithium and sodium  are  found to be same since, sodium is the eighth element after Lithium.


 

 

Newlands law of octaves limitations:

                          or
what are the limitations of newlands law of octaves?

i) The first limitations which came out is that the law doesn't applicable to the elements after calcium, i.e. after calcium every eighth element doesn't posses similar properties to that of the first.

 

ii)  The second limitation of it was that, according to Newland's  only 56 element exist in nature& also stated that no more element could be discovered in future.But later on many more elements discovered and in addition to that those new elements properties doesn't fit with Newland's Law of Octaves.

 

iii)The third limitations of this law was that, in order to fit the elements in the table, Newland's  put two elements in the same slot, in addition to that he also put unlike element under the same note.

 

  Mendeleev periodic table :

Mendeleev Periodic Table

 After the failure of Newland's Law of Octaves,  a Russian chemist Dmitri Ivanovich  Mendeleev tried to classify elements in 1872.  At that time only 63 elements were known.Mendeleev periodic table is based on" atomic Masses of the elements and also on their similarities of chemical properties".

 In mendeleev periodic table explanation, he examined atomic masses and physical and chemical properties of the elements. For chemical properties, he took hydrogen and oxygen as they are very reactive in nature and forms compounds with most of the elements.In 2nd row of the periodic table, he put the formulae of hydrides and oxides formed by an element. Mendeleev put  all the 63 elements in such a way that they are in order of increasing atomic masses. The table contains vertical columns called "groups" and horizontal rows called "periods".


Mendeleev periodic law:

             or

Mendeleev law:

Mendeleev Periodic Law states that "the properties of element in a periodic table are the periodic function of their atomic masses".

  

Limitations Of Mendeleev Periodic Table:  

 i) The first limitation of it, was that no fixed position was given to hydrogen.

ii)  The second limitations were that the isotopes were discovered after the classification of Mendeleev Periodic table(isotopes are those elements which have similar chemical properties but different atomic masses). 

 

Modern periodic table:

 

the modern periodic table

After the failure of Mendeleev periodic table,the modern periodic table is organized by a scientist  Henry Moseley in 1913 explained that atomic number is more fundamental property which should be placed in place of atomic mass in periodic table. Hence,Mendeleev Periodic Table was modified on this basis with few more changes stated below.

 

 modern periodic table characteristics:

 i) The first change that make in modern periodic table is that it is consist of 18 vertical columns called "groups" and 7 horizontal rows known as "periods".

 ii) The second  change, is that the anomalous position of hydrogen was solved, and it is placed in first group & first period.

iii) The third change within it, is that the elements  in each groups are arranged  in such a way that they contain same number of valence electron.

iv)The periods are arranged in such a way that the number of valence  electrons  increases by one unit, as the atomic no increases by one unit, by   moving  from left to right in the period.

v) The atomic radius decreases for the elements moving from left to right along the period.This is due to the fact that  moving from left to right along periods, increases the nuclear charge  which tends to pull the electron closer to the nucleus and thus it reduce the size  of the electron.


vi) Down the groups a new shell is added to the electronic  configuration of the elements, which increases the distance between the outermost electron and nucleus, thereby increasing the atomic size down the group.

vii) The metal elements(sodium, aluminium,etc) are present on the left side of the periodic table, while the non-metals(sulphur,chlorine, etc) are present on the left side of it and between this two there is a zig-zag line of elements which are semi-metal or metalloids (Boron, Arsenic, etc)which separate metals from non-metal.

 

Miscellaneous questions from the above discussion:
 
1) Modern periodic table was given by which scientist?

Ans:- It was given by Henry Moseley in 1913 as a  modification of Mendeleev periodic table, by taking atomic number as fundamental property of chemical properties in place of atomic mass.

2)How is the modern periodic table arranged ?

Ans:-It was arranged with 18 groups and 7 periods, with the  position of hydrogen in 1st group and 1st period.


3) Who was father of modern periodic table ? 
                        or 
who made modern periodic table ?
 
Ans:-Dmitri Ivanovich  Mendeleev 
 
 
4) mendeleev's periodic table of elements was organized by ?
 
Ans:-Dmitri Ivanovich  Mendeleev   in the year 1872.
 
 5)mendeleev periodic table arrangement is based on which factor? 
 
Ans:- Atomic mass.

 

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