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History of Periodic Table

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History of Periodic Table - Lesson Summary

The earliest classification categorized elements into metals and non-metals. It was difficult to classify the elements, such as boron, which exhibited the properties of both metals as well as non-metals. After further research a German scientist, Dobereiner arrived at a hypothesis in the year 1829.



According to Dobereiner, all elements occurred in groups of three, when arranged in increasing order of atomic masses. He referred to these groups as triads. In a traid the elements had similar chemical properties. 

Traids of the Dobereiners classification:
Traid1:

Element Atomic mass Lithium  (Li) 7 Sodium (Na) 23 Potassium (K) 39


Traid2:

Element Atomic mass Chlorine (Cl) 35.5 Bromine (Br) 80 Iodine (I) 127


Traid3:

Element Atomic mass Calcium (Ca) 40 Strontium (Sr) 88 Barium (Ba) 137


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. The atomic mass of lithium is 7 and  the atomic mass of potassium is 39. The average of masses of lithium and potassium gives atomic mass of sodium 23. 

Drawbacks:
All the known elements could not be arranged in the form of triads.
This law did not hold good for elements with very low or very high atomic mass. 
Example: The arithmetic mean of the atomic masses of fluorine 19 and bromine 80, which comes to 49.5, varies significantly from the atomic mass of chlorine, which is 35.5. 

Since Dobereiner’s law could not successfully group elements, the attempts at classification continued. The next attempt came in 1864, when an English chemist, John Newlands, stated his observations in the form of Newlands Law of Octaves.

Newlands Law of Octaves:



When Newlands arranged elements in according to their atomic weights then there was similarity of every eighth element. Newland described it as "law of octaves".
According to this law every eighth element is similar to that of the first element, similar to the first and the eighth notes in the musical scale. 

Newlands classification of elements:

Li Be B C N O F Na Mg Al Si P S Cl K Ca
Example: When starting from lithium, the eighth element is sodium. Similarly, the eighth element from sodium is potassium. Lithium, sodium and potassium show similar properties.

Drawbacks:
  • Not valid for elements having atomic masses higher than calcium.
  • Newly discovered elements could not find a place in Newlands table.

Mendeleev's periodic table:



Mendeleev based his work on the research by Newlands and took it further. He felt that effective grouping of elements and prediction of properties could be based on two parameters:
  • Atomic mass 
  • Chemical reactivity

Mendeleev’s periodic law states that "the physical and chemical properties of all elements are the periodic function of their atomic masses".

  Row  Group I  Group II  Group III  Group IV  Group V  Group VI  Group VII  Group VIII  1
 2
 3 H = 1
Li = 7
Na = 23 Be = 9

Mg = 24 B = 11

Al = 27
C = 12
Si = 28
N = 14
P = 31
O = 16
S = 32
F = 19
Cl = 35.5  4

 5 K = 39

Cu = 63 Ca = 40

Zn = 65 Ti = 48 V = 51

As = 75 Cr = 52

Se = 79 Mn = 35

Br = 80 Fe = 56
Co = 58.94
Ni = 58.68  6

 7 Rb = 85

Ag = 108 Sr = 87

Cd = 112 Y = 89

In = 113 Zr = 90

Sn = 118 Nb = 94

Sb = 120 Mo = 96

Te = 128

I = 127 Ru = 103
Rh = 104
Pd = 106  8
 9 Cs = 133 Ba = 137 La = 138 Ce = 140  10

 11

Au = 198

Hg = 200 Yb = 173

Tl = 204

Pb = 206 Ta = 182

Bi = 208 W = 184 Os = 191
Ir =193
Pt = 196  12  Th = 232  U = 240
The main features of Mendeleev’s periodic table:
  • The known 63 elements were classified into groups and periods.
  • The table had 8 vertical columns called groups and 12 horizontal rows called periods.
  • In every group, a gradation of physical and chemical properties of elements was observed.
  • The table provided gaps for undiscovered elements.
  • The table helped to predict the properties of three unknown elements of that time. These elements were named eka-boron, eka-aluminium and eka-silicon. When these elements were discovered, they were named scandium, gallium and germanium. The properties of these elements were very close to those predicted by Mendeleev.

Merits:
The table helped in the correction of atomic mass for many elements. It predicted the existence of some elements that have not been discovered at the time the table was created.

Demerits:
The atomic weights of two pairs of elements were reversed. 
Alkali metals and coinage metals were placed in the same group. 
Lanthanides and actinides were not given proper place in the periodic table. 
Isotopes were not placed in the periodic table. 
The position of hydrogen was not clearly discussed

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