Page 134 - Chemistry ICSE Class IX

122 ICSE Chemistry – 9
ƒ Prediction of the properties of undiscovered elements. All the
undiscovered elements were later discovered and found to have
properties similar to those predicted by Mendeleev. For example,
scandium (Sc), gallium (Ga) and germanium (Ge) discovered later
had properties similar to those predicted for eka-boron, eka-aluminium
and eka-silicon, respectively.
The predicted and the observed properties of eka-silicon (germanium)
are presented in Table 5.3.

Table 5.3 Mendeleev’s predictions for eka-silicon and the properties of germanium

Predicted properties of Winkler’s report of
Property
eka-silicon (1871) germanium (1886)
1. Atomic mass 72 72.6
Germanium 2. Density 5.5 5.36
3. Melting point, K High 1231
4. Action of acid Likely to be slightly Not attacked by HCl
attacked Reacts with hot HNO 3
5. Action of alkali Likely not to react No action with dil. NaOH
6. Oxide MO (4.7) GeO (4.7)
2
2
7. Sulphide MS 2 GeS 2
8. Chloride MCl (1.9) GeCl (1.88)
4
4
ƒ Correcting the incorrect atomic masses of some elements. By placing
elements strictly according to the similarity in their properties,
Mendeleev was able to correct certain atomic masses. For example,
the atomic masses of beryllium (Be), gold (Au) and platinum (Pt)
were corrected by Mendeleev.

What are the drawbacks in Mendeleev’s periodic table
Mendeleev’s periodic table was a brilliant attempt for the classi cation of
elements, but it had certain drawbacks in it.
ƒ Inversion in the periodic table. In Mendeleev’s table, certain pairs
of elements were placed in the reverse order of atomic masses. For
example, cobalt (Co) having higher atomic mass had been placed
before nickel (Ni) having lower atomic mass. Also, argon (40) was
placed before potassium (39). This contradicted Mendeleev’s periodic
law.
ƒ Position of isotopes. Isotopes have similar chemical properties, but
different atomic masses. According to Mendeleev’s periodic law,
therefore, isotopes of an element must be given separate places in
the periodic table. But, isotopes were not given separate places in the
periodic table.
ƒ Grouping of chemically dissimilar elements. Certain chemically
dissimilar elements have been grouped together in Mendeleev’s
table. For example, Cu and Ag have no resemblance with alkali metals
(Li, Na, K, etc.), but these have been grouped together in Group I.
ƒ Separation of chemically similar elements. Certain elements which
appear to be chemically similar (like Cu and Hg, Au and Pt, etc.),
have been placed in separate groups.
ƒ Position of hydrogen. Hydrogen forms both positive ion like alkali
metals and negative ion like halogens. Thus, hydrogen resembles both
alkali metals and halogens. Therefore, it can be placed both in Group I
and Group VII. In Mendeleev’s table, the position of hydrogen was
not made clear.