Question

If Li, Na, and an unknown element X, form a Döbereiner's triad, then the atomic mass of X is ______ amu.

• A. 39

Answer 1

The correct option is A 39
In a Döbereiner's triad, the atomic mass of the middle element is roughly the average of the atomic masses of the other two elements.

Calculation:

Atomic mass of Li = 7 amu
Atomic mass of Na = 23 amu
Let the atomic mass of the element X be Z.
Since Li, Na, and X form a Dobereiner's triad,

$\Rightarrow \frac{(7+Z)}{2}=23$
$\Rightarrow 7+Z=46$
$\Rightarrow Z=39$

So, the atomic mass of X is 39 amu.