The question asks about Dobereiner's Law of Triads, a foundational concept in the early classification of elements. This law describes a specific mathematical relationship between the atomic masses of three chemically similar elements grouped into a "triad". We need to identify the correct mathematical relationship.
This formula represents the arithmetic mean of the atomic masses of the first and third elements.
A) The arithmetic mean of the atomic masses of the first and third element. This directly matches the definition and mathematical formulation of Dobereiner's Law of Triads.
Dobereiner's Triads were an early attempt to classify elements based on their properties. According to Dobereiner's Law of Triads, when elements are arranged in groups of three (triads) in increasing order of their atomic masses, the atomic mass of the middle element is approximately the arithmetic mean of the atomic masses of the other two elements. We need to check each option against this rule.
C) Li (6.9), Na (23), K (39.1) — mean of 6.9 and 39.1 \(\approx\) 23. This option correctly demonstrates Dobereiner's Triad as the elements have similar chemical properties and the atomic mass of the middle element (Na) is the arithmetic mean of the other two (Li and K).
The question asks about John Newlands' Law of Octaves. This law was an early attempt to classify elements based on their properties. We need to recall the core principle of this law to identify the correct statement.
Correct Option: B) Every eighth element has properties similar to the first element, when arranged in order of increasing atomic mass. This statement accurately describes John Newlands' Law of Octaves, highlighting both the periodicity (every eighth element) and the basis of arrangement (increasing atomic mass).
The question asks for the main drawback of Newlands' Law of Octaves. We need to recall the limitations of Newlands' classification system for elements.
Correct Option: C) It worked only for lighter elements up to calcium and failed for heavier elements; also, no provision was made for undiscovered elements.
The question asks about Mendeleev's Periodic Law, specifically what property of elements it uses as the basis for periodicity. We need to recall the historical context and the fundamental principle behind Mendeleev's original periodic table.
Correct Option: D) Atomic masses
The question asks to identify the element that Mendeleev predicted as 'Eka-silicon' and which was later discovered, filling a gap below silicon in his periodic table. This requires knowledge of Mendeleev's periodic table and his predictions.
C) Germanium (Ge), which Mendeleev called 'Eka-silicon' — Mendeleev indeed predicted an element he called 'Eka-silicon' based on its position below silicon. This element was later discovered and named Germanium (Ge), and its properties closely matched Mendeleev's predictions.