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PERIODIC CLASSIFICATION OF ELEMENTS (MENDELEEV'S PERIODIC LAW…
PERIODIC CLASSIFICATION OF ELEMENTS
DOBERNIER'S CLASSIFICATION
Tried to classify and group elements in increasing order of their atomic masses
Made group of 3 elements(triads) having similar chemical properties
Mass of the middle element was equal to the average mass of the other two elements
Masses: 6.9,22.9,39 According to 3rd condition 6.9+39=45.9/2=22.95
DRAWBACKS OF DOBERNIER
could only make 3 triads
At that time only few elements(52) were known,the grouping was not done for all elements so it was not accepted
NEWLAND'S OCTAVES
Grouped seven elements
Arranged them in increasing order of their atomic masses
Started with the lowest element hydrogen and ended with thorium the 56th element known at that time
Every eight element showed similar properties as the first.called the OCTAVES(like the music notes 8th similar to 1st
DRAWBACKS OF OCTAVES
Could only group the elements only till calcium,after Ca the remaining elements did not follow the law of octaves
Newland assumed that only 56 elements existed in nature and no new element will be discovered
He tried to jumble few elements to make possible that the law of octaves is followed i.e the properties of the 1st and 8th element is same
In order to fit elements in octave,adjusted 2 elements,cobalt and nickle by placing in same group of fluorine,chlorine and bromine and iron which resembled cobalt and nickel placed in some other group
MENDELEEV'S PERIODIC LAW
Properties of elements are periodic functions of their atomic masses
Mendeleev arranged elements in increasing order of their masses and elements lying in the group have same properties
63 elements known at that time
Made a table of 8 groups(vertical columns) and 6 periods(horizontal rows)
ACHIEVEMENTS
While arranging elements Mendeleev left 3 gaps-- predicted 3 elements as EKA BORON, EKA ALUMINIUM,EKA SILICON
Also predicted properties of these elements
All 3 elements were discovered later named as a)SCANDIUM b)GALLIUM c)GERMANIUM
Noble gases not discovered then.After discovery got separate group without disturbing elements in table
Noble gases are inert gases---less reactive & are present in very less concentration in air
LIMITATIONS/ANOMALIES
While arranging elements in table,placed cobalt(58.9) before Nickel(58.7).
Position of Hydrogen not clear because was placed along with alkali metals.------->could have been placed along with fluorine,Chlorine ,Bromine and Iodine(halogen family) because it forms di-atomic molecule like them it combines with metals forming covalent bonds00
Isotopes not given any position in table.Isotopes are atom of same element with same atomic number but diff atomic mass e.g., 35/17 CL
