Tuesday, December 27, 2011

X CBSE | NCERT Solutions Periodic Classification of Elements E- notes (Dobereiner Trieds)

OBJECTIVES OF THIS LESSON Periodic Classification of Elements
After completing this lesson, you will be able to:
! state different historical classifications of elements in brief;
! state main features of Mendeleev’s periodic table;
! explain the defects of Mendeleev’s periodic table;
! state modern periodic law;
! describe the features of the long form of periodic table;
! define various periodic properties;
! discuss the trends in various periodic properties in the periodic table.

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 These elements are called Halogen
Alkalies Earth metal as they react with metal and form salt .
as their oxide are alkalies in nature
and exist in the Earth
Drawback: 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 his work The Royal Society , London awarded By Davy Medal in 1887
Limitation:
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.
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