Study the data of the following three categories A, B and C.

Category	Name of the element	Atomic Mass
A	Li	7
	Na	23
	K	39
B	N	14
	P	31
	As	74
C	B	10.8
	Al	27
	Ga	69.7

(i) From the given three categories A, B and C, pick the one which forms Dobereiner's Triads.

(ii) Why did Mendeleev placed elements of category A, B and C in three different groups?

(iii) Is Newland law of octaves applicable to all the three categories? Give reason to justify your answer.

(i) Dobereiner's Triad

From the given categories, Category A ($\text{Li}$, $\text{Na}$, $\text{K}$) forms a Dobereiner's Triad.

Justification:

According to Dobereiner's law, when elements are arranged in order of increasing atomic masses, the atomic mass of the middle element is roughly the arithmetic mean of the atomic masses of the other two elements.

$$\text{Arithmetic Mean of Li and K} = \frac{\text{Atomic Mass of Li} + \text{Atomic Mass of K}}{2} = \frac{7 + 39}{2} = \frac{46}{2} = 23$$

This is exactly equal to the atomic mass of Sodium ($\text{Na}$), which is $23$.

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(ii) Mendeleev's Placement in Different Groups

Mendeleev placed the elements of categories A, B, and C in different groups because they exhibit entirely different chemical properties, valencies, and oxide characteristics:

• Category A ($\text{Li, Na, K}$): These are highly reactive alkali metals with a valency of $1$. They react with oxygen to form basic oxides (e.g., $\text{Na}_2\text{O}$, $\text{K}_2\text{O}$) which dissolve in water to form strong alkalis ($\text{NaOH}$, $\text{KOH}$).
• Category B ($\text{N, P, As}$): These are non-metals/metalloids with a typical valency of $3$ or $5$. They react with oxygen to form acidic oxides (e.g., phosphorus pentoxide) which dissolve in water to form acidic solutions.
• Category C ($\text{B, Al, Ga}$): These are Group 13 elements with a valency of $3$. Aluminium, for instance, forms an amphoteric oxide ($\text{Al}_2\text{O}_3$) that reacts with both acids and bases.

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(iii) Applicability of Newlands' Law of Octaves

No, Newlands' Law of Octaves is not applicable to all three categories.

Reason:

Newlands' Law of Octaves was found to be applicable only to lighter elements up to Calcium (atomic mass $\approx 40$). Beyond Calcium, every eighth element did not possess properties similar to those of the first. In Category B, Arsenic ($\text{As}$, atomic mass $74$) and in Category C, Gallium ($\text{Ga}$, atomic mass $69.7$) lie far beyond Calcium, meaning the law of octaves breaks down and does not apply to these categories.