CBSE NCERT Class 10 Science chapter 5
Chapter 5 – Periodic Classification of Elements For Class 10 Download pdf
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Döbereiner's Triads (1817), Johan Wolfgang Döbereiner:
[CBSE-2020]
First to classify elements into triads.
When the three elements in triad were written in the order of increasing atomic masses, the Atomic mass of the middle elements was roughly the average of the atomic masses of the other
two elements.
Atomic mass Ca 0
Sr, Na
Br
Na
23
7+39 23
K
Bal
1
K
40+137 88.5
40 87
137 Ba > Döbereiner made the first observation on platinum as a catalyst and discovered similar triads of element which led to the development of periodic table
of elements.
[NCERT]
Limitations of Döbereiner Triads:
Identified only three triads from the elements known at time.
Large number of similar elements could not be grouped into triads It was possible that quite dissimilar elements could be grouped in a triads.
Q. Can the following groups of elements be classified as Döbereiner's triad?
i) Na, Si, CI
ii) Be, Mg, Ca
[CBSE - 2019]
Atomic mass of Be - 9, Na-23, Mg-24, Si-28, Cl-35, Ca- 40 Justify your Answer.
Sol. 1) Na, Si, Cl have different properties, therefore they do not form Döbereiner's triads even though the atomic mass of the middle atom Si is approximately the average of the Atomic masses of Na and CI..
Na (23)
SI (28)
CI (35)
Atomic mass of Si = 23+35-29
ii) Be, Mg, Ca have many similar properties and also the atomic mass of the middle
element Mg is approximately the average of the atomic masses of Be and Ca
Be (g);
Mg(24);
Ca(40)
9440
49 = 24.5
2
2
Newland's Law of Octaves (1866, John Newlands)
1. Newland arranged the known elements in order of increasing atomic masses and found that
every eight element had properties similar to that of the first.
2. He compared this to the octave found in the musical notes, therefore, He called it the "law of octaves" 3. Properties of Li and Na were found to be the same sodium is the eight element after Lithium.
Sa ga ma
Da
Ni
l
C
(ca)
N
(ti)
0
H
Li
Be
B
F
Na
Mg
Al
Si
P
S
CI
K
Ca
Cu
Ti
Mn
Fe
Co & Ni
Cu
Zn
Y
In
As
Se
Br
Rb
Sr
Ce and La
Limitations of Newland's Octave:
[NCERT]
1. It was found that the law of octave was applicable only up to calcium, as after calcium every 8 element did not possess properties similar to that of the first. [Exemplar]
2. After discovery of Nobel gases, it become difficult to fit them in Newland's periodic table.
3. It was assumed by Newlands that only 56 elements existed in nature and no more elements would be discovered in the future, But later on several new elements were discovered, whose properties did not fit into the law of octave,
4. In order to fit elements into his table, Newlands adjusted two elements in the same slot. 5. Co, Ni are in same slot and these are placed in the column as F., Cl, Br.
6. Fe, which resembles Co and Ni in properties, has been placed far away from these elements.
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