Mendeleev's Periodic Classification
Dmitri Ivanovich Mendeleev, a Russian scientist arranged the elements in increasing order of their relative atomic masses. He was honoured with Noble prize in 1906 for his Periodic Table.
Mendeleev's Periodic Law states that the properties of elements are the periodic function of their relative atomic masses.
Mendeleev arranged all 63 elements (which were discovered till his time) in the order of their increasing relative atomic masses in a tabular form. It is known as Mendeleev’s Periodic Table. He divided the table in eight columns and seven rows. The columns are known as groups and rows are known as periods.
Explanation of Mendeleev's Periodic Table:
- Elements are arranged in the periodic table in the increasing order of their relative atomic masses.
- Mendeleev divided his periodic table in eight groups and seven periods.
- Groups from I to VII are meant for normal elements and group VIII is for transition elements.
- Groups from I to VII have been divided in two sub groups, while group VIII is meant for three elements.
- Periods from 4th to 7th have been divided in two series: 1st series and 2nd series.
- Elements having similar properties have been kept in the same group. For example, lithium, potassium, rubidium, etc. are in 1st group.
Two general formulae, one for oxides and second for hydrides, have been given for the elements of each group in the periodic table. For example: R2O for oxides and RH for hydrides, of the elements; of 1st group.
Using the given general formulae; the formula of oxides and hydrides can be written for the elements of each group. For example; hydrogen, sodium, potassium, etc. belong to the first group. The general formula of oxides for the elements of 1st group is R2O. Accordingly they form H2O, Na2O, K2O, etc.
Merits of Mendeleev's Periodic Table:
Mendeleev left some blank spaces in his periodic table in order to place the elements having similar properties in the same group.
For example, titanium has been placed in IVth group, leaving a blank space adjacent to it in IIIrd group. Similarly, arsenic has been placed in Vth group; leaving two adjacent spaces blank. These spaces have been occupied by scandium, gallium and germanium after their subsequent discovery.
Prediction for the elements to be discovered in future:
Mendeleev predicted the discovery of some elements and named them as eka-boron, eka-aluminium and eka-silicon. He gave the name of these elements prefixing the word ‘eka’ to the name of the preceding elements.
Scandium, Gallium and Germanium have been discovered later and took the place of eka-carbon, eka-aluminium and eka-silicon, respectively in the gap left in the Mendeleev’s Periodic table; as their properties were exactly similar to the predicted elements.
Position of Noble gases which were discovered later:
Noble gases were discovered much later after Mendeleev. After the discovery of noble gas, they were placed in a separate group called Zero Group, after VIII group, without making any disturbance to the arrangement of any elements in the Mendeleev’s Periodic Table. Noble gases are chemically un-reactive and present in very low concentration in the atmosphere.
Limitation of Mendeleev's Periodic Table
Position of Hydrogen: Hydrogen has been placed in 1st group with alkali metals, since hydrogen makes compound in the same way as alkali metals do.
On the other hand, hydrogen exists as diatomic molecule; similar to halogen and hydrogen makes covalent compounds also as halogens do.
Thus, on the basis of properties of hydrogen similar to halogen, hydrogen may be placed with halogens but Mendeleev did not explain about this anomaly.
Position of Isotopes: Elements having same atomic number but different atomic masses are known as isotopes. Although isotopes were discovered after Mendeleev, but it became a challenge to accommodate those isotopes in Mendeleev’s Periodic Table without disturbing the order of elements.
Wrong Order of Elements: Mendeleev placed many elements in wrong order of their increasing atomic masses in order to place elements having similar properties in similar group.
Example: The atomic mass of nickel is less than that of cobalt, in spite of that cobalt is placed before nickel. The atomic mass of Chromium is 50.20 and the atomic mass of vanadium is 50.94. In spite of this, chromium is placed after vanadium.
In spite of above limitations and anomalies, the Mendeleev's Periodic Table was one of the wonderful discoveries.