Why ??
Why is there a need of classification ?
To make an order out of chaos , to notice a pattern/similarities among elements & ofcourse to increase a chapter in our book 📚

➡️ Atomic mass(middle [2nd] element) = Average of Atomic mass (first element) & Atomic mass (second element)

Beech wale element ka atomic mass baki dono ka average hai

➡️ Only a limited number of elements were classified and hence it was not useful
➡️ Classification was done only on the bases of atomic masses & not Properties of the elements

Search for the table in book which has the Newlands Octaves

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Drawbacks

➡️ It was applicable only upto Calcium as after that every 8th element did not have similar properties to the first one .

➡️ Newlands considered that there were only 56 Elements in nature and didnt leave any space for elements to be discovered in the future.

➡️ Cobalt & Nickel are in the same slot and are placed with fluroine , chlorine & bromine which have completely different properties & Iron which has similar properties to them is placed far away in the table 😂

1 para ki beizzati for Newlands

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Mendeleev studied the relation between atomic mass and the physical/chemical properties of the elements
At his time only 63 Elements were known .

He came up with his "periodic table" which is still called Modern Periodic Table . Thus , his role was very significant in formation of the Table . see the Table in NCERT !

ACHIEVEMENTS OF MENDELEEVS PEIODIC TABLE

➡️ He placed some elements before others even if they had higher atomic masses so that elements with similar properties could be grouped together.

➡️ He left some gaps in his table & predicted that some elements which are not discovered yet will be placed here .He named these yet to be discovered elements as :- Eka-Boron = scandium , Eka-Aluminum  = gallium, Eka-Silicon = germanium

➡️ When inert gases were discovered and added in his table , it didn't disturb the position of other elements which was an extraordinary achievement

Drawbacks Of Mendeleev's Table

  ➡️ Hydrogen had similar properties to both alkali metals and Halogens , hence they can't be placed in the column with alkali metals . Clearly no place could be given to hydrogen & hence he could not give a proper position to it in the table.

➡️ Isotopes (same atomic number but different atomic masses) could not be placed in the table as it would completely disturb it . This is the reason modern periodic table uses atomic number as a base for classification.

 ➡️ Atomic Masses do not increase by a fixed number in elements . They increase in an irregular manner.

Koi kitna bhi acha kam karle NCERT wale drawbacks de dete hai . Book ke end me NCERT padne ke drawbacks bhi hone chahiye . Who agrees ? Like thoko aur apni screen tod do

Change from Mendeleev's table : Elements are a periodic function of their atomic numbers not masses. It overcame the drawbacks of Mendeleev's Table like isotopes placement , and elements' atomic number always changes by 1 and not an irregular number as in cases of atomic masses .

About the MODERN PERIODIC TABLE

 1) It has 118 elements.


 2) It has 18 vertical columns (groups)& 7 horizontal rows (periods).


 3) Periods have elements with same number of shells .


 4) Groups have elements with same number of valence electrons.

Some Random Things

♦️ Hydrogen has no "appropriate" position in the Periodic Table because it has properties similar to  Alkali metals as well as Halogens

♦️ Number of Electrons which can come in a shell is given by the formula 2n² where n is the shell number . So for example shell 2 i.e. L Shell will have 2(2)² = 8 electrons

Trends in Periodic Table (Most imp topic)

                ♣️ VALENCY

 As we move from left to right in periodic table , the valency increases . This means that across a period (horizontal row) , the valency increases

                ♣️ ATOMIC SIZE 

The atomic radius decreases from left to right across a period . It is because as the number of protons increase , the EFFECTIVE NUCLEAR CHARGE also increases which causes the electrons to come closer to the nucleus and thereby reduce its size .

    ♣️ Metallic & Non Metallic properties 

Metallic charcter is the willingness or rather the ease with which an atom loses an electron & Non Metal is the exact opposite of it . As we move from top to bottom , the metallic character increases & non-metallic charcter decreases . Why ? Because the number of shells in the atom increase as we go towards the bottom and hence , the EFFECTIVE NUCLEAR CHARGE (The attraction between protons and valence electrons) decreases and so the atom loses the electron easily . 

Also , Metallic Character decreases & Non metallic charcter increases from left to right because the number of valence electrons increases and hence , the ABILITY to take electrons also increases.  

Note : Non metallic character is same as electronegativity (ability to take electrons) and Metallic character is same as electropositivity (ability to lose/give electrons)