Question

A.why do we need to classify elements?

B.explan Genesis of periodic classification?(In point wise)

Answer 1

A)The number of elements discovered has increased, making it difficult to remember the behaviour and properties of these elements. Hence it is important to classify elements according to their properties. It is also necessary to understand how they form the compounds.
Grouping of elements into different classes is called periodic classification of elements.
This method requires arranging the elements that are alike and separating the elements that are unlike to compare the properties of elements.
It helps us understand how different elements form different compounds.

B) Dobereinier’s Triads:



He discovered that there is certain similar elements that existed in a group of 3 elements. He named them ‘Triads’.



In the group of 3 element i.e. triads, the atomic mass of the middle element is the arithmetic mean of atomic masses of other 2 elements.



Example: Li, Na and K belong to one triad.



Element Atomic mass

Li 7

Na 23

K 39

7+39/2 = 23



Law of Octaves:



J.A. Newlands placed the elements in ascending order of atomic masses and found that the 8th element starting from the given one is a form of repetition of the 1st element.



Law of octaves cannot be applied beyond calcium. Also, with the discovery of noble gases, the 8th element is not anymore a similar element.