Dobereiner’s triads were groups of elements with similar properties that were identified by the German chemist Johann Wolfgang Dobereiner. He observed that groups of three elements (triads) could be formed in which all the elements shared similar physical and chemical properties.
Dobereiner stated in his law of triads that the arithmetic mean of the atomic masses of the first and third element in a triad would be approximately equal to the atomic mass of the second element in that triad. He also suggested that this law could be extended for other quantifiable properties of elements, such as density.
(a) No, since, despite the fact that the atomic mass of silicon is the average of the atomic masses of sodium (Na) and chlorine (Cl), none of these elements has identical properties.
(b) Yes, because they have similar properties and magnesium’s mass (Mg) is about equal to the average of Be and Ca’s atomic masses.