Page 131 - Chemistry ICSE Class IX

The Periodic Table 119
In these triads, the atomic mass of the middle element is nearly equal
to the mean of the rst and third elements in each triad. Dobereiner could
arrange only a few elements out of 63 elements known at that time, in the
form of triads. Although, he got some success but the idea of classi cation
into triads could not be applied to all the elements. So, the idea of classifying
the elements into triads was abandoned.

What is Newlands’ law of octaves
In 1864, John Newlands, an English chemist and a musician, arranged the
elements known at that time, in the order of their increasing atomic masses.
He observed that “the properties of each element resembled those of the
eighth element before it and of the eighth element following it.”
In other words, he found that “the properties of the elements were
repeated at every eighth element like the eighth note of an octave in
music.”
The First and the Eighth Notes are Identical
1 2 3 4 5 6 7
Sa Re Ga Ma Pa Dha Nee
Li Be B C N O F
Na Mg Al Si P S Cl
K

From the arrangement given above, it is observed that every eighth
element has similar properties, i.e., Li, Na and K have similar properties.
Newlands called this as the law of octaves.
The law of octaves worked quite well for lighter elements, i.e., for
elements, such as lithium, sodium and potassium.
Like musical notes, elements were
arranged in octaves
Why was Newlands’ law of octaves discarded
Newlands’ law of octaves was discarded due to the following reasons:
ƒ It did not work well for heavier elements, i.e., for the elements lying :ŽŚŶ EĞǁůĂŶĚƐ ǁĂƐ ŚŽŶŽƵƌĞĚ ďLJ ƚŚĞ
beyond calcium. ZŽLJĂů ^ŽĐŝĞƚLJ ǁŝƚŚ ƚŚĞ ĂǀLJ DĞĚĂů
ƒ After the discovery of noble gases, the concept of octaves failed because ŝŶ ϭϴϴϳ ĨŽƌ ŚŝƐ ĐŽŶƚƌŝďƵƟŽŶƐ ƚŽǁĂƌĚƐ
then the ninth element and not the eighth showed similar properties. ƚŚĞ ĐůĂƐƐŝĮĐĂƟŽŶ ŽĨ ĞůĞŵĞŶƚƐ͘

Illustrative Questions

Q.1. Why was the necessity of classifying the elements into certain groups felt?
Ans. With the discovery of more and more elements and their compounds, it became very dif cult to study and
remember the properties of all these individually. So, a need to classify them into certain groups was felt.
Q.2. X, Y and Z are three members of Dobereiner’s triad. The atomic mass of X is 7 u and that of Y is 23 u. What is the
atomic mass of Z?
Ans. The triad is X, Y, Z. Let the atomic mass of Z be x. Then, according to Dobereiner,
Atomic mass of X + Atomic mass of Z
Atomic mass of Y =
2
7 u + x
23 u =
2
This give, x = 2 × 23 u – 7 u = 46 u – 7 u = 39 u
Therefore, the atomic mass of Z is 39 u.