Monday, September 26, 2011

X CBSE | NCERT E- Notes For Ch - Periodic Classification of Elements of Elements

Classification of elements: the arrangement of elements in such a manner that elements with similar properties are grouped together while elements with dissimilar properties are separated.

EARLY ATTEMPTS OF CLASSIFICATION OF ELEMENTS
1. Lavoisier’s classification of elements: In 1789, Lavoisier first attempted to classify the elements into two divisions namely Metals and Non-metals. However this classification was not satisfactory as there were many exceptions in each category.
2. Dobereiner’s classification of elements: In 1817, Johann Wolfgang Dobereiner grouped three elements into what he termed triads.
In each case, the middle element has an atomic mass almost equal to the average atomic masses of the other two elements in the triad.
For example, elements like lithium, sodium and potassium have atomic masses 7, 23 and 39 respectively. They are grouped together into a triad as,
Li (7) Na (23) K (39)
Here the atomic mass of sodium is the average of atomic masses of lithium and potassium.
Limitation of Dobereiner’s law
Only three triads were identified from the element known at that time. Hence, this classification was not useful.
Newland’s law of octaves:              Read more »


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