Question

Assertion; Elements $Li,Na$ and $K$constitute a triad.

Reason: Atomic mass of Na is nearly mean of the atomic mass of $Li$ and $K.$

Mark the correct choice as

• A. Both assertion and reason are true and the reason is the correct explanation of assertion.
• B. Both assertion and reason are true but the reason is not the correct explanation of assertion.
• C. Assertion is correct but the reason is incorrect.
• D. Assertion is incorrect but reason is correct

Answer 1

The correct option is A

Both assertion and reason are true and the reason is the correct explanation of assertion.

Explanation for assertion statement:

1. J W Dobereiner found that certain elements have similar properties and they could be arranged in a group of three called triads. Triad 1 was,

Element	Lithium	Sodium	Potassium
Atomic masses (g/mol)	6.94	22.99	39.1

Explanation for reason statement:

1. Triad was made in such a way that the atomic mass of the middle element was approximately equal to the arithmetic mean of the atomic mass of the other two elements in the triad.
2. Mass of $Sodium\left(Na\right)=\frac{massofLithium+massofpotassium}{2}$

$$\begin{gathered} =\frac{6.94+39.1}{2} \\ =23.02g/mol \end{gathered}$$

Hence, A) Both assertion and reason are true, reason is the correct explanation of assertion.