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