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Introduction And Historical Background Of Periodic Table



Introduction 
To achieve a thorough understanding of a complex subject like chemistry.It would be highly desirable to fit all the facts into a simple logical pattern. The periodic table of elements has served the purpose to systematize the properties of the elements for well over 100 years .The development of periodic table is one of the most significant achievements in the history of chemical sciences.
Periodic Table
A table obtained by the arrangement of elements into periods and groups is called periodic table

Usefulness of periodic table
The periodic table provides a basic framework to study the periodic behavior of physical and chemical properties of elements as well as their compounds.

Historical Background
The early history of ideas leading up to the periodic classification of elements is fascinating, but will not be treated here in detail. Those who made memorable contributions in this field are:
AL- RAZI
He organized the known elements and compounds. His classification was based upon the physical and chemical properties of elements and compounds.
Dobereiner’s  Triads
In 1829, a Germen chemist, Dobereiner arranged them known elements in groups called Triads, as each contained three elements with similar properties. According to his law of Triads:
“ The atomic mass of middle element is average of atomic masses of other two elements of triads”
Some Dobereiner’s  Triads
Triad                  Li                  Na                     K

Atomic Mass      7                  23                      23
Atomic  mass of Na = 7+29 /2= 23
Triad                  Ca                 Sr                      Ba

Atomic Mass     40                 88                        137
Atomic mass of Sr= 40+137/2= 88.5
Triad                Cl             Br                   I

Atomic Mass       35.5          80                        127
Atomic mass of Br = 35.5+127/2 = 81.25

Demerits
Dobereiner‘s  ideas of classification failed because only few elements could be covered under triads.

Newlands’s  Law of Octaves
In 1864 an English chemist, john Newland classified 62 elements, known at that time, in increasing order of their atomic masses. According to his law of Octaves:
“If the elements are arranged in the increasing order of their atomic masses, every 8th elements had some properties in common with

Newland’s Octaves
Li (7)

Be (9)
C (12)
N (14)
O (16)
F (19)
B (11)
Na (23)

Mg (24)
Al (27)
Si (28)
P (31)
S (32)
Cl (35.5)
K (39)


Ca (40)
Ti (48)
Cr (52)
Mn (55)
Fe (56)
Ni & Co

Demerits
1: It was found that the  law of Octaves  was applicable only up to calcium , as after calcium every eighth element did not posses properties similar to that of the first.
2: It was assumed by Newland that only 62 elements existed in nature no or more elements would be discovered in the future. But , later on, several  new elements were discovered. Whose properties did not fit into the law of Octaves.
3: The above sequence of elements was disturbed with the discovery of noble gases.

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