Can the following groups of elements be classified as Dobereiner's triad:
`{:("(a) Na, Si, Cl"," (b) Be, Mg, Ca"):}`
Atomic mass of `Be-9, Na-23, Mg-24, Si-28, Cl-35, Ca-40.` Justify your answer in
each case.

Explanation

Na, Si, Cl - The properties of these three elements are not similar to each other, so no Dobereiners triad.
Be, Mg, Ca - The properties are similar to each other so it is Dobereiner.s triad.
Atomic mass of Mg = `("Atomic mass of Be + Atomic mass of Ca")/2`
=24.5