Question

Class-X
3. 30 Assertion(A). Li, Na and K forms a Dobereiner triads because the atomic mass of the middle
element is roughly the average of the atomic mass of the other two elements. I
Reason(R): Dobereiner arranged elements on the basis of increasing atomic number.
A) Both A and R are true and R is correct explanation of the A.
B) Both A and R are true but is not the correct explanation of the A
C) A is true but R is false,
D) A is false but R is true.​

Answer 1

Answer : Option "C" is correct

Explanation:

Dobereiner arranged the elements on the basis of increasing atomic mass

Answer 2

The correct answer about Dobereiner Triad:  A is true but R is false .

Explanation:

Dobereiner Triad classification

"According to it, when elements are arranged in order of increasing atomic masses groups of three elements have similar chemical properties are obtained".  

In this atomic mass of middle element is arithmetic mean of atomic masses of other two elements.  

e.g.  

Li  Na K

 7      23     39  (Alkali gp.)  

Atomic mass of sodium = 7+39/2 =23

Similarities shown :

 They have some chemical prop. i.e.  

• all are metals  

• all react with water to form alkalis  

• all have valency-1

Similarly ,another triad :

Cl  Br I

 35.5  80     127 halogen gp.  

Similarities shown :

• All are non-metals

• All react with water of form acids

• All have valency1

At mass of Br. = 35.5 + 127 /2 = 80

Limitation of this classification

He failed to arrange all known elements in form of triad because he couldn't found more groups or triads with above said similarities .