👉The arrangement of all the known element according to their properties in such a way that the elements of similar properties are grouped together in tabular form is called Periodic Table.
A).
i.)Lavished classified the element simply in metals and non metals.
-> Metals are the one which have the tendency of losing the electrons.
-> Non-Metal are the one which have the tendency of gaining the electrons.
ii.) Drawbacks
a.) As the number of element increased this classification became insufficient for the study of element.
b.) There are few element which have the properties of both metals as well as non metals and they are called metalloids.
- Lavoisier could not decide where to place the metalloids.
B.) Prout's Hypothesis ( Unitary theory )
- He simply assumed that all element made of Hydrogen, so can say that.
[ Atomic weight of element = n×Atomic weight of Hydrogen. ]
Where n= number of Hydrogen atom= 1,2,3.........
Drawback
I.) Every element cannot be formed by Hydrogen. Ex:-Chlorine:- 35.5.
II.) Atomic weight of all element were not found as the whole number.
C.) Dobereiner Triad Rule (1817)
I. He made group of three element having similar chemical properties called Triad.
Ex:- A,B,C (element)
Atomic weight B = A+C/2
Li = 7
Na=?
K=39
7+39/2=23
Na=23
-> Element from same group.
Note= Dobereiner Triad apply only group.( do not period )
Delta Z => Same
Delta Z=> Atomic no. Most be same differences of
Ex. Atomic no
Drawback
1. Not valid for all
Ex. F Cl Br
19 ? 80
19+80/2= 49.5
Newland Octave Rule (1865)
He arranged the elements in the increasing order of their Atomic masses and observed that properties of every 8th elements was similar to the 1st element.( like in the case of musical vowels natation. )
Drawback:-
I.Valid up to Ca.
2.Failed after discovery of Noble Gas.
E. Lother Meyer's Curve (1869)
I. He plotted a curve between Atomic weight and Atomic volume of different elements.
Observations.
a.) Most electropositive element i. e alkali metals ( Li,Na,K,Rb,Cs etc.) (IA group) occupy the peak position on the curve.
b.) Less electronegative i.e alkaline earth metal ( Be,MG,Sr,Ba)(II A group) occupy the descending position on the curve.
c.) Metalloids (Si,As,Te,etc.) And transition metal occupy bottom part of the curve.
d.) Most electronegative :- halogen ( F,Cl,Br,I) occupy the ascending position on the curve.
Mendeleev Periodic Table
• The Physical and chemical properties of element are Periodic function of Atomic weight.
• First scientist which classify the elements in horizontal row and vertical column.
• 63 element are known
• Horizontal row is called Period. There are 6 period.
• Vertical Column is called Group. There are 8 Group.
• Each group divided in 2 subgroup. Up to VII Group.
• Group A ( Normal element) s&p- block element, expept zero gas,Noble Gas.
• Group B ( Transition element )
Merits of Mendeleev Periodic Table
A). Study of element.
B). Prediction of new element (Ga, Sc, Ge, Tc)
C). Correction of doubtful Atomic weight.
☆. Atomic weight = Valence× Equivalent weight.
Eka Aluminum = Galuium.
Eka Boron = Seandium.
Eka Silicorna = Germanium.
Demerits of Mendeleev Periodic Table
a). Position of Hydrogen.
b). Position of isotopes.
c). Anomalous pairs of element