Question

*In Dobereiner's triad, 23 is the mean of atomic masses of Li and K. This is the atomic mass of ____.*

1️⃣ Na
2️⃣ Ca
3️⃣ Br
4️⃣ I​

Answer 1

Na

Solution: The atomic mass of sodium is equal to the sum of the atomic masses of Li and K, or '6.9 + 39.1)/2 =23'.

Sodium will have an atomic mass of 23.

Answer 2

Answer:

1. Na

Explanation:

• According to Dobereiner, in a given triad, the (arithmetic) mean of the atomic masses of the first and third element is approximately equal to that of the second element in the respective triad.
• The first triad - Li, Na, K
  The second triad - Ca, Br, Sr
  The third triad - Cl, Br, I
  The fourth triad - S, Se, Te
  The fifth triad - Fe, Co, Ni
• Now, in the first triad (Li, Na, K)
  the atomic mass of Li is 7 u
  the atomic mass of K is 39 u
• The average - $\frac{7 + 39}{2}$ = 23 u, which has to be the atomic mass of the second element in the triad, i.e., Na.
  
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