(a) State Newland Law of Octaves.

(b) With an example, explain Dobereiner's Triads.

(c) List one limitation each of both the attempts mentioned in 'a' & 'b'.

(a) Newlands' Law of Octaves:

When elements are arranged in the order of increasing atomic masses, the properties of every eighth element are a repetition of the properties of the first element, similar to the octaves of music (where the eighth note is the same as the first).

(b) Dobereiner's Triads:

Johann Wolfgang Döbereiner classified elements with similar chemical properties into groups of three, known as triads. When these elements are arranged in increasing order of their atomic masses, the atomic mass of the middle element is approximately equal to the arithmetic mean of the atomic masses of the other two elements.

• Example: The triad of Lithium ($\text{Li}$), Sodium ($\text{Na}$), and Potassium ($\text{K}$).
• Atomic mass of Lithium ($\text{Li}$) = $7.0\text{ u}$
• Atomic mass of Potassium ($\text{K}$) = $39.0\text{ u}$
• Average mass = $$\frac{7.0 + 39.0}{2} = 23.0\text{ u}$$

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

(c) Limitations:

• Limitation of Newlands' Law of Octaves: It was applicable only up to Calcium ($\text{Ca}$). Beyond Calcium, the eighth element did not possess properties similar to those of the first.
• Limitation of Dobereiner's Triads: Dobereiner could identify only three triads from the elements known at that time, making this system of classification highly limited and not universally applicable.