Final answer:
Dobereiner's classification was limited by its inability to group all elements into triads and was too simplistic to address the discovery of new elements. The related triad system was based on properties such as atomic weight and reactivity. In contrast, Newlands' Law of Octaves faced criticism for its arbitrary placement of elements and failure to accommodate elements heavier than calcium.
Step-by-step explanation:
The limitations of Dobereiner's classification were that it was not able to accommodate all known elements within triads, and it could not predict the discovery of new elements. Dobereiner organized elements into groups of three, or triads, such as lithium, sodium, and potassium, based on their chemical properties and relative atomic mass. However, this system had shortcomings because many elements did not fit into this triadic pattern. Moreover, with the discovery of more elements, it became clear that the triad system was too simplistic to address the complexities of element properties and relationships.
Dobereiner believed that lithium, sodium, and potassium belonged in a triad because the properties of sodium (atomic weight and chemical properties) were an average of lithium and potassium. The triad system was based on properties such as atomic weight, reactivity, and other chemical characteristics.
Contrast with Newlands
John Newlands proposed the Law of Octaves, which also used atomic weight to organize elements, with every eighth element (starting from a given one) having similar properties to the first. However, this law did not hold true for heavier elements beyond calcium, and it faced criticism for the arbitrary placement of elements to make the octave pattern fit.