explain Mendeleev's periodic classification

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1. Dmitri Ivanovich Mendeleev, a Russian scientist arranged the elements in increasing order of their relative atomic masses.

2. Mendeleev’s Periodic Law states that the properties of elements are the periodic function of their relative atomic masses.

3. 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.

4. The explanation of Mendeleev's Periodic Table is given below-

• 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.

4. 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: R 2 O for oxides and RH for hydrides, of the elements; of 1st group.

5. 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 R 2 O. Accordingly they form H 2 O, Na 2 O, K 2 O, etc.

6. The merits of Mendeleev's periodic table are-

• Mendeleev left some blank spaces in his periodic table in order to place the elements having similar properties in the same group.
• 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.
• 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.

7. The limitations of Mendeleev's Periodic table are-

• 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.
• 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.
• Mendeleev placed many elements in wrong order of their increasing atomic masses in order to place elements having similar properties in similar group.
  

Best Wishes.

Hello Aspirant,

In 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: R 2 O 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 R 2 O. Accordingly they form H 2 O, Na 2 O, K 2 O, etc.

• He arranged the elements based on their atomic masses.
• He observed that when the elements were arranged in increasing order of their atomic masses, there was a periodic occuring again in their physical and chemical properties.
• Thats why he formulated a Periodic Law, which states that the properties of elements are the periodic function of their atomic masses.
• Mendelevs Periodic Table contains vertical columns called groups and horizontal rows called periods.
• He also left some gaps which has not been discovered.