Question

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.
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 R 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:

i think option( c ) ....

plz ..tell me if it is wrong

Answer 2

For the given question assertion is true while reason is false. So correct option is (C).

Dobereiner arranged the elements according to increasing atomic masses and not according to their atomic number hence reason is false.

The elements were grouped in group of three known as triad. The elements in a triad had similar properties  and the atomic weight of middle element is close to the arithmetic mean of other two elements in triad.

For Li,Na and K we have:-

Atomic weight of Li=7 and that of K=39 and Na=23

As weight of sodium = (weight of Li + weight of K)/2

Weight of sodium = 7+39/2 = 46/2 = 23 which is weight of Na

Hence assertion is true.

So correct answer is option (C)