Question

The atomic masses of three elements $A$, $B$ and $C$ having similar chemical properties are $7$, $23$ and $39$ respectively.
(a) Calculate the average atomic mass of elements $A$ and $C$.
(b) Compare the average atomic mass with atomic mass of $B$.
(c) What could the elements $A$, $B$ and $C$ be?

Answer 1

(a) The average atomic mass of elements A and C $=\frac{7+39}{2}$ $=\frac{46}{2}$ $=23$
(b) The average atomic mass of elements A and C is the same as the atomic mass of element B=23.
(c) The element A is Lithium (Li), B is Sodium (Na) and C is Potassium (K). They are Dobereiner's Triads because in Dobereiner's Triads the atomic mass of the middle element is approximately the mean of the atomic masses of other two elements.