Mendeleev's Classification

In March 1869 Dimitri Mendeleev a Russian chemist arranged the elements in order of increasing atomic mass, placing the elements with similar chemical properties vertically beneath each other. By doing so, he observed that the properties of the elements with slight modification repeated themselves at intervals. So he put forward his Periodic Law which states that.

"The physical and chemical properties of elements are a periodic function of their atomic weights."

The periodic table published by Mendeleev consisted of eight vertical columns called groups (i.e. Group I to VIII) and horizontal rows called series. Now-a-days these series are called periods. The table is shown below:

Table 4.3 Mendeleevís Periodic Table of 1872

ROW

 

Group I

Group II

Group III

Group IV

Group V

Group VI

Group VII

Group VIII

1

H = I

 

 

 

 

 

 

 

2

Li=7

Be=94

B11

C = 12

N = 14

0=16

F= 19

 

3

Na=3

Mg=24

Al=27.3

Się 28

P=31

S=2

CI - 35.5

 

4

K=39

Ca=40

_____ = 44

Ti = 48

B = 51

Cr=52

Mil = 55

Fe=6,Co=59

Ni=9,Cu=3

5

(Cu=3)

Zn=65

____=68

_____ -72

As = 75

Sc=8

Br = 80

          

6

Rb = 85

Sr=7

?Yt = 88

Zr = 9Q

Nh = 94

Mo=96

ó =100

JRu=104, Rh =04

Pb = 106,Ag=108

7

(Ag=108)

Cd=112

In =113

Sn=118

Sb=122

Te=25

1 = 127

 

8

Cs= 133

Ba= 137

?Di=138

?Ce = 140

 

 

 

 

9

 

 

 

 

 

 

 

 

10

 

 

 

 

 

 

 

?Er= 178

?La= 180

Ta=182

W=184

 

Os = 195,Ir=197 Pt = 198, Au = 199

11

(Au =199)

Hg = 200

Tl = 204

Pb = 207

Bi = 208

 

 

 

12

 

 

 

Th = 231

...

U = 240

 

 

 

Spaces were left for the unknown elements with atomic masses 44, 68, 72 and 100.

 

Salient Features of Table:

 

1.    It has eight vertical columns called groups and twelve horizontal rows called periods.

2.    Elements in each vertical column have similar properties.

3.    Vacant spaces were left for the elements not discovered until then. He proposed their names as eka-boron, eka-aluminium and eka-silicon.

4.    He group number indicate the highest valence that can be attained by elements of that group.

Advantages of Mendeleev's Periodic Table:

1.    It helped in systematic study of elements. For example the study of sodium helps means to a large extent in predicting the properties of other alkali metals as potassium, rubidium, cesium. It forcefully proved the concept of periodicity.

2.    Prediction of new elements was made possible. The physical and chemical properties of eka-boron, eka-aluminum and eka-silicon were predicted by Mendeleev. This helped in their discovery. These have been named as scandium (Sc), Gallium (Ga) and Germanium (Ge). Their properties are remarkably the same as were predicted by Mendeleev.

3.    Mendeleevís periodic table helped in correcting many doubtful atomic Masses.

INTERESTING TO NOTE

Discovery of Gallium and Confirmation of Mendeleev's Prediction

Gallium was unknown when Mendeleev developed his periodic table Mendeleev left many vacant spaces in his periodic table, stating that such vacant spaces would be filled by elements not yet discovered. He did not only left vacant spaces but he also predicted the properties of the elements] which when found, would occupy these spaces, like those, eka (below)aluminum and silicon, bi 1875, when the French Chemist Paul Emile Lecoq, discovered the new element Gallium (from Latin Gallia for France) and gave it density as 4.7g/cm3. Mendeleev pointed out that the density should be 5.94g/cm3, that was proved true as predicted. The scientific community was astounded that Mendeleev (Theorist) knew the properties of new element better than the chemist who has discovered it. The new element Gallium which was called by Mendeleev Eka-Aluminum, so named because it occupied the first vacant space in his periodic table under aluminum.

Mendeleev's periodic table proved to be the key for unlocking the mysteries of atomic structure and chemical bonding. In the nineteenth century, element 101, was named Mendelevium (Md) in his honor.

Defects In Mendeleev's Periodic Table:

1.    There are three pairs of elements i.e. elements of higher atomic masses placed before elements of lower atomic masses i.e.

a.    Argon (40) placed before potassium (39)

b.    Cobalt (59.9) placed before nickle (58.6)

c.    Tellurium (127.6) placed before iodine (126.9)

2.    No place for isotopes of elements.

3.    Dissimilar elements placed in same group i.e. Alkali metals (Li, Na, K, Rb, Cs, Fr) were placed with coinage metals (Ag, Cu, Au)

4.    Similar elements placed in different groups for example Barium (Ba) and leads (Pb) resemble in many properties but they are placed in separate groups.

5.    It failed to give the idea of atomic structure.


 

 
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