Lithium, sodium and potassium were put in one group or triad on the basis of their similiar properties. What is the name of this group? State their properties.

Lithium, sodium and potassium formed a group called alkali metal group. Their similar properties are:
(i) All these elements are metals.
(ii) All these react readily with water to form alkalies and hydrogen gas.
(iii) All of them have a valency of 1.

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Name the triads that Dobereiner could suggest?

Döbereiner suggested the following triads:
(i) Alkali metal family—Li, Na, K
(ii) Alkaline earth family—Ca, Sr, Ba
(iii) Halogen family—CI, Br, I

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What made scientists to think of classification?

As early as the begining of nineteeth century, it was found that certain elements resembled with other elements in certain aspects. For example, flourine, chlorine, bromine and iodine were found to show similarity in many properties.
 
In this way scientists tried to group elements with similar properties so that the study of all the known elements and their compounds could be made with the help of a few groups only.
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Why elements need to be classify?


There are one hundred and fourteen elements and each element forms a large number of compounds. It is, therefore, necessary to understand the basis of their properties and the rules according to which these react. This is possible if the elements are classified and grouped together based on their properties.

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Consider the following elements:
K, I, Li, Sr, CI, Mg, Br, Ca, Na.
Separate these elements in three groups or triads having similar properties. State one property in each case on the basis of which groups are separated.


These elements can be separated into three groups as given below:
(a) K, Li, Na—have valency of (+1).
(b) Sr, Mg, Ca—have valency of (+2).
(c) I, CI, Br—have valency of (-1).

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