Periodic Classification Of Elements

PERIODIC CLASSIFICATION OF ELEMENTS

JEE mains and NEET

 

·     Classification -Classification means grouping of metal on the basis of similarities in property.

 

Need of Classification-

It is difficult to study of each and every element individually and to known it properties and uses therefore they have been classified into gtoups on the basis of their similarities in properties.

Basis of classification –

On the basis of similarities in properties .

ð   Dobereiner’s Law of Triads

 

When the elements are arranged in the group of three inincreasing of atomic masses .The middle element of a group has the atomic masses and property roughly the average of other two elements .These elements shows similarity in their property.

Eg.

Li     Na     K

7      23     39             7+39/2=46/2=23

 

2)Cl       Br         I

   35.5   80        127

35.5+127/2=81.25

 

Importants:

The above classification of experiments into Triads as a great significance in producing atomic mass and the property of middle element.

 

Limitation:

All the element could not be classified into Triad discovered at that time .In 1800 ,30 elements were known.

 

     Newland’s Law of Octave

Newland arranged the element in 1800 in increasing order of atomic masses ,properties of 8 element are repeatation of property of first element.

Limitation:

All the element discovered at that time could not be arranged into octate.

 

Arrangement:

The need of arranging element into vertical coloumn called groups.Horizontal group called periods.Thus,become evident and indoing so,periodicity in their  properties could be observed.

 

 

Periodicity of Property:

Repeatation of similar property after a definite interval is called Periodicity in Properties.

 

                   Mendeleev Classfication

Mendeleev Periodic Table was published in 1872.Mendeleev arranged 63 element known at that time on the basis of their similarities in their properties that is on the basis of their similarities ,formula of their oxides ,hydrates at the top of the coloumn later is use to represent element of the group formed by O and H i.e RO and He observed that most of the element where placed in the increasing order of atomic masses.It was also observed that there is a reccurence of similar physical and chemical properties of element after a regular interval of elelment every 8 th element.

Mendeleev Periodic Law

Mendeleev formulated a periodic law which state that “The properties of element are the periodic function of their atomic masses.”

Mendeleev Periodic table contains vertical coloumn called group (8 group) and horizontal rows called period (7 period) out of 8 group first 7 are of normal element and 8 th group is of transition element .Noble gasses where known at that time so,No group of noble group was provided in  Mendeleev periodic table:

1)Increase order of atomic masses.

2)Grouping together of element having similar properties .

 

ü Merit of Mendeleev Periodic Table:

1)Mendeleev Predicted the existence of some element Mendeleev left proper gap for undiscovered element like eka Boron ,eka aluminium,and eka silicon[meaning of eka is one]

So eka boron means first come boron and then unknown element (later named Scandium Sc),when these elements discovered later on then eka -Boron.

2)Mendeleev Periodic could not predict the properties of several elements on the basis of their position in periodic table.The Properties of undiscovered element like Barium ,Scandium and Germanium where predicted in this way.

^{3)Mendeleev Periodic table could accommodate noble} gasses when they were discovered when the noble gases where discovered it got place in the periodic table in the form of  a separate group with disturbing the original arrangement of Mendeleev  Periodic table.

 

ü Limitation of Mendeleev Table:

1)The position of Mendeleev periodic table isotpes are the same element having similar chemical properties but different atomic masses ..if the element are arranged according to the atomic masses isotope should be placed in different group of periodic table because they different atomic masses .Isotope were not given separate places in Mendeleev periodic table they were kept at the same place for eg.Chlorine,Cl,Cl has two isotopes.Cl³⁵,Cl^37.

2)Wrong of atomic masses of same element could not explained certain element where put if the current group on the basis of their chemical properties it was found that element with high atomic mass comes first and element with lower atomic masses later .eg-cobalt was put in the correct group on the basis of chemical property but cobalt having higher atomic mass  of 58.9

Comes first and nickel having atomic mass of 58.7 comes later .Mendeleev could not explain the wrong order of atomic masses .

3)A correct position of hydrogen could not be assigned ,hydrogen has been placed in group one with alkali metal this is because alkali metal hydrogen also combined with halogen (chlorine ,bromine, Iodine).

Hydrogen is also rejumbled halogen in some properties Hydrogen is also exist in H₂ ,Moreover just like halogen .Hydrogen also combine with certain metals to form ionic compound  called hydrides and reactb with non-metal to covalent compound.Due to these properties hydrogen also placed in group 7 of halogen element.

 

 

 

ü    MODERN PERIODIC TABLE

·      Henry Moseley an English scientist found that atomic number was the fundamental property of an element and not the atomic mass for the classification.

 

Modern  Periodic Law

 

“Properties of an element are the periodic function odf their atomic number .

Modern Periodic Table-Basis of classification of MPT is increasing order of their atomic number.

All the drawback of Mendeleev Periodic table abolished when the element are arranged on the basis of increasing atomic number.

ð Main features of MPT

1)It shows arrangement of element based on modern Periodic law.

2)There are 18 vertical coloumn known as group.

3)There are seven horizontal coloumn rows known as periods.

Period	No of element	Name
1	2	Very shot
2	8	Short period
3	8	Short period
4	18	Long period
5	18	Long period
6	32	Very long period
7	Rest of the element	It is incomplete

1)First Period contains two element Hydrogen and Helium .They have only one shell .

2)Second Period contain eight elements having two shell that is K and L shell.

 

 

Period

 There are seven period in MPT