This is part:-1
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Introduction:- (by present)
You know that there are 118 elements are known to us.And all elements have different properties. Out of these 118, only 94 are occurring in nature.
*Now from starting of classification of elements
As different elements were being discovered scientists collecting more and more information about properties of these elements.
They found that it makes difficulty to organise all known properties about these elements. They started looking for search some pattern in their properties of these elements to make chart of properties of these elements.
1. Arranging Chaos (Chaos:- a State of great disorder, confusion) one of the first attempt of classification of elements.
We have seen learn how various living things all living beings classified according to properties. for example in shops soaps are kept together. While biscuits are kept together even among biscuits same flavours of biscuits stacked separately from different other flavour of biscuits.
Similarly, scientists made several Attempts to classify elements on the basis of their property and to obtain an orderly arrangemant out of Chaos.
The earliest first attempt to made orderly arrangement of elements results in grouping in two, one is metals other is non metal of then known elements at that time.
But later more new elements were discovered with different properties that could it shows all create more confusion to arrange properly inorder on the basis of their properties of elements.
1.1 DOBEREINER’S TRIADS
A day in the year of 1817 Johann Wolfgang DOBEREINER a German chemist scientist tried to arrange the elements into groups on the basis of their similar properties. After that you know that some groups of elements with similar properties having three elements each. so he called those group’s named as triads. DOBEREINER say that when taking the elements in triads in increasing then the atomic mass of middle of the triads is approximately equal to the average atomic masses of other two elements in the triad.
For example, take the triad consisting of lithium (Li) sodium(Na) and potassium(K) with atomic masses 6.9 23.0 and 39.0. what is the average atomic masses of Li and K? i.e.(39+6.9)/2=approximately 46/2=23.
How does this compare with the atomic mass Na? conclusion is atomic masses of middle element was roughly the average of the atomic masses of other two elements.
NCERT QUESTIONS:-
Given below (table 1) are some groups of three elements. These elements are arranged downwards in order of increasing atomic masses.
Can you find out which of these groups from dobereiner triad?
TABLE 1
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| Write answer in the comment box. |
You will find that groups B and B form Dobereiner triads. Dobereiner could identify only three triads from the elements known at that time(Table 2). The system of classification into triads was not found to be useful.
Table 2
