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Tuesday, December 27, 2011

X Periodic Classification of Elements I Dobereiner Triads I Newland’s law of Octaves

OBJECTIVES OF THIS LESSON Periodic Classification of Elements

The first classification of elements was as metals and non-metals. This served only limited purpose mainly because of two reasons:

1. All the elements were grouped in to these two classes only. The group containing metals was very big.


2. Some elements showed properties of both-metals and non-metals and they could not be placed in any of the two classes.


Dobereiner’s triads  :  In 1829, Dobereiner, a German scientist made some groups of three elements each and called them triads.

He observed that the Atomic mass of the middle element of a triad was nearly equal to the arithmetic mean of atomic masses of other two elements.

Element
symbol
Atomic mass
Lithium,
Li
7
Sodium,
Na
23
Potassium,
K
39

These elements are called Alkalies metal as they react with metal and form alkalies (Caustic solution)


Element
symbol
Atomic mass
Element
symbol
Atomic mass
Calcium
Ca
40
Chlorine,
Cl
35.5
Strontium,
Sr
88
Bromine,
Br
80
Barium,
Ba
137
Iodine,
I
127
These elements are called Alkalies Earth metal  as their oxide are alkalies in nature and exist in the Earth
These elements are called Halogen  as they react with metal and form salt .

Drawback of Dobereiner’s triads   Dobereneir’s idea of classification of elements into triads did not receive wide acceptance as he could not arrange all elements in this manner.


Newland’s law of Octaves : In 1864 John Alexander Newland, an English chemist noticed that “when elements are arranged in the increasing order of their atomic masses. He found that "every eighth element had properties similar to the first element.”

Newland called it the Law of Octaves. It was due to its similarity with musical notes .


Look carefully at the Newland’s arrangement of elements shown below:
Li
Be
B
C
N
O
F
(6.9)
(9.0)
(10.8)
(12.0)
(14.0)
(16.0)
(19.0)
Na
Mg
Al
Si
P
S
Cl
(23.0)
(24.3)
(27.0)
(28.1)
(31.0)
(32.1)
(35.5)
K
Ca





(39.1)
(40.1) 





Because  this work The Royal Society , London awarded By Davy Medal in 1887

Limitations of the Law of Octaves

1.Newland could arrange elements in this manner only up to calcium (atomic mass 40 )out of a total of over sixty elements known at his time.

2. No Place  given for forth coming elements.

3. He placed unlike elements in same slot like Co and Ni placed with F,Cland Iodine

4. When Nobel gas were discovered in 1900 law of octave fail and 9th element became similar to 1st.

Now read this topic
Mendeleev’s periodic table

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