Question

49 pts...
State Döbereiners's law of triads and its limitations.
State Newlands' Law of Octaves and its limitations.​

Answer 1

$\huge\underline\mathfrak\red{Dobereiner's\:law\:of\:triads}$

In the year 1817, a German chemist Johann Welfgang dobereiner attempted to classify the elements. He arranged the elements in the groups of 3 each. The groups of three elements were known as triads.

He showed that if the three elements in a triad were written in order of increasing atomic masses then the atomic mass of middle element was roughly the average of the atomic masses of the other two elements.

Example : There was a triad of Lithium, sodium and potassium.

Atomic mass of Lithium is 6.94,

Atomic mass of sodium is 23.04,

Atomic mass of potassium is 39.04.

According to dobereiner's rule, when these elements are arranged in order of their increasing atomic masses, the mass of sodium comes to be an average of atomic masses of Lithium and potassium.

Example :

Atomic mass of Na = $\dfrac{Atomic\:mass\:of\:Li\:and\:K}{2}$

=> $\dfrac{7+39}{2}$

=> 23.0

which is a true quantity.

Similarly, he observed other triads of elements like :

Ca, Sr, Ba and Cl, Br, I

He also followed same rule as Li, Na and K.

Limitation :

Dobereiner could identify only three triads from the elements known at that time. In other words, he could arrange only 9 elements out of about 30 elements. So his triad rule was not applicable to all the elements. Hence, his system of classification was neglected due to non usefulness of his system of classification.

__________________________

$\huge\underline\mathfrak\red{Newla d's\:law\:of\:Octaves}$

After dobereiner's failure, other scientists classify the elements by correlating the properties of elements with their atomic masses.

In a similar attempt, in 1866, an England scientist, John Newlands, arranged elements known at that time in order of their increasing atomic masses. At that time 56 elements were known. He arrange the elements in order of their increasing atomic masses. He started the arrangement of elements from the element of lowest atomic mass, the hydrogen and ended at the element having maximum atomic mass, the thorium which was 56th element at that time.

He found a uniqueness in this type of arrangement that every eighth element had the properties similar to that of the first. He compared this to the octaves found in music system. Hence, he called this the law of octaves. This law is also known as the newlands law of octaves. In this system, properties of Lithium and sodium were found to be same. Sodium is 8th element at that time.

He compare the octaves of musical system with his classification. There are 7 notes in Indian musical system, every 8 not repeats the first one.

Sa

Re

Ga

Ma

Pa

Dha or Da

Ni

Limitations :

★ This law was applicable only upto calcium. After this element every 8th element did not posses properties similar to the first.

★ It was assumed by newlands that only 56 elements existed in nature and no more elements would be discovered further in future. But later, several elements were discovered whose properties did not fit into the law of octaves.

★ In order to fit elements according to similar properties he adjusted unlike elements in same slot, eg cobalt and nickel are kept in same column as chlorine, flourine and bromine. These elements have different properties yet placed in same slot which is an anomaly. Some elements have same properties, yet place in different columns, which is also not convenient.

Hence, this law is applicable only for lighter elements and not for all, so rejected!

Answer 2

Answer:-

Dobereiner's law of triads

Dobereiner's law of triads states that, the atomic mass of the middle element of a triad is the arithmetic mean of the atomic masses of the other two elements.

Example:

In the triad of lithium, sodium and potassium.

All the known elements could not be arranged in the form of triads.

______________________________

Limitations:-

• All known elements could not be classified into groups of triads on the basis of their properties. Not all groups obeyed the Law of Triads.

For example, nitrogen family does not obey the law of triads.

✔️ Dobereiner could identify only three traids from the element known at that time.

_______________________________

Newlands' law of octaves

According to Newlands' law of octaves

when the elements are arranged in order of increasing atomic weights found that every eighth element had properties similar to that of the first.

He compared these to the octaves found in music. Therefore, he called it is the '

Law of octaves'.It is known

as 'Newlands'law of octaves'.

Example:-

Chromium (Cr) and yttrium (Y) are the first and the eighth element placed in the same column but they have entirely different properties.

______________________________

Limitations:-

• 1) Newlands assumed that only 56 elements existed in nature and no more elements would be further discovered in the nearer future.

• 2)In order to fit elements into law of octaves Newlands not only adjusted two elements
• in the same slot but also adjusted some unlike elements under the same note.

• 3)Cobalt and nickel are in the same slot and are positioned in the same column with fluorine, chlorine and bromine possessing different properties than these elements
• 4)Iron possessing similar properties as cobalt and nickel, is placed far away from these elements.

• 5) Thus, Newlands’ Law of Octaves can be obeyed well by the lighter elements only