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Mendeleev's Periodic Table


In March 1869, Mendeleev gave his famous scheme of the periodic classification of elements. "It states that the properties of the elements are the periodic functions of their atomic weights.

Mendeleev arranged all the elements known at that time in horizontal rows in order of increasing atomic weights. He left some gaps in the table for undiscovered elements and also predicted their properties. Eventually, when these elements were discovered, it was amazing to find that they fitted correctly in the table. For example, as can be seen in Mendeleev's original periodic table there are two gaps between zinc (group II) and arsenic (group V). As these undiscovered elements were to follow aluminium and silicon, Mendeleev named them eka-aluminium and eka-silicon. When these elements, now known as gallium (eka-aluminium) and germanium (eka-silicon), were discovered, their properties were agreed very well with those given by Mendeleev.

Mendeleev's prediction for eka-aluminium (Gallium)

Property

Eka-aluminium (Ea) predicted in 1871 by Mendeleev

Gallium (Ga)

Reported in 1875

Currently accepted

Atomic mass/amu

68

69.9

69.72

Density, r/ /g cm-2

5.9

5.94

5.904

Melting point, T/K

Low

303.15

302.78

Solubility in acid and alkali

Ea will dissolve slowly in both acid and alkali

Ga dissolves slowly in both acid and alkali

Ga dissolves slowly in both acid and alkali

Formula of oxide

Ea2O3

Ga2O3

Ga2O3 ((a and b forms)

Density of oxide r /g cm-2

5.5

-

6.44 ((a form) and 5.883 (b form)

Reactions of sulphate

Ea2(SO4)3 will form alum

Gallium forms alums

Gallium forms alums, e.g. (NH4)2SO.Ga (SO).24HO

Preparation of Sulphide

Ea2S3 will be precipitated by H2S or (NH4)2S

Ga2S3 is precipitated by H2S or (NH4)2S

Ga2S3 is precipitated by H2S or (NH4)2S

Properties of chloride

EaCl3 will be more volatile than ZnCl2

GaCl3 is more volatile than ZnCl2

GaCl3 is more volatile than ZnCl2


Mendeleev's prediction for eka-silicon (Germanium)

Property

Eka-silicon (Es) predicted in 1871 by Mendeleev

Germanium(Ge)

Reported in 1886 by Bioshaudran

currently accepted

Atomic mass/ amu

72

72.32

72.59

Density, /g cm-3

5.5

5.47

5.35

Melting point, T/K

High

-

1220

Specific heat capacity, c/J g-1

0.3051

0.3177

0.3903

Molar Volume, Vm/ cm3

13

13.22

13.5

Colour

Dark grey

Greyish white

Greyish white

Valence

4

4

4

Reaction with acids and alkalis

ES will be slightly attacked by acids but will resist attack by alkalis

Ge is dissolved by neither HCl nor NaOH, but dissolved by conc. NaOH

Ge is dissolved by neither HCl nor NaOH, but dissolved by conc. NaOH

Boiling point of tetraethyl derivative, T/K

433

433

458-460

Density of the dioxide, /g cm-3

4.7

4.7

4.2

Density of the tetrachloride, /g cm-3

1.9

1.887

1.844

Boiling point of the tetrachloride, T/K

373

359

357





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