According to Newlands law of octaves, which of the following was the last element in his classification?

Introduction / Context:
This question belongs to the history of the periodic table and specifically to Newlands law of octaves. Before the modern long form periodic table was developed, several scientists tried to arrange elements in order of their properties. John Newlands suggested that when elements are arranged in order of increasing atomic mass, every eighth element has similar properties, similar to musical octaves. Knowing which elements he managed to include gives insight into the limitations of his scheme.

Given Data / Assumptions:

• We are dealing with Newlands classification based on the law of octaves.
• Four candidate elements are listed: rhodium, radium, iron, and thorium.
• We need to identify which element was the last included in his sequence.
• Assume typical school level historical description of Newlands work.

Concept / Approach:
Newlands arranged known elements of his time in increasing order of atomic mass and noticed that every eighth element had similar properties. However his law worked only reasonably well up to a certain point because many elements were not yet discovered and heavier elements did not follow his pattern. Historical records and common exam references mention that Newlands arrangement extended up to thorium. Elements beyond this were either unknown or did not fit his scheme, which contributed to criticism of his work.

Step-by-Step Solution:
Step 1: Recall the range of elements known to Newlands. When Newlands proposed his law, only about 56 elements were known. Step 2: Recognise from standard teaching that his table was extended only up to thorium. Many textbooks state that thorium was the last element included in Newlands octaves arrangement. Step 3: Compare with the options provided. Rhodium, radium, and iron are not recorded as the terminal element in his arrangement; iron appears earlier in the series. Thorium appears at the end of the sequence he managed to classify under his law.

Verification / Alternative Check:
The correctness can be cross checked from standard history of chemistry notes, which usually mention that Newlands tried to fit known elements into his octave pattern until thorium. Beyond thorium, the regularity broke down badly, and he did not have space for future discoveries, which was a major limitation of his classification. No standard reference mentions rhodium or radium as the last element in his octave series, reinforcing that thorium is the expected answer in typical exam questions.

Why Other Options Are Wrong:
Option A: Rhodium is a transition metal that was not highlighted as the last element under Newlands law and does not appear as the terminal element in standard discussions of his table. Option B: Radium is an element that was discovered much later than Newlands time and therefore could not have been part of his classification. Option C: Iron is found earlier among the elements and is not treated as the final element of the octave pattern. Option D: Thorium is widely stated as the last element considered in Newlands octaves, so this is the correct choice.

Common Pitfalls:
One frequent confusion is between Newlands law of octaves and Mendeleev periodic table. Students may also mix up which scientist dealt with which range of elements. Another mistake is to guess based on atomic numbers or metallic character instead of remembering the historical fact. Keeping a short timeline of Dobereiner triads, Newlands octaves, and Mendeleev periodic law helps keep these contributions distinct in memory.

Final Answer:
The last element in Newlands law of octaves classification was Thorium.