Mendeleev’s Periodic table

Introduction to Mendeleev's Periodic Table in Chemistry

The Periodic Table created by Dmitri Mendeleev is a significant achievement of science, which changed the views on the elements of the matter. First unveiled to the scientific community in 1869, Mendeleev’s table was a radically new approach compared to other prior attempts. In other words, the concepts that Mendeleev developed when arranging the elements in order of increasing atomic weight and formulating some of the principles of his tables also involved the establishment of some periodic tendencies that enabled him not only to classify known elements of the time but also work out the predictable characteristics of yet undiscovered elements. His ideology brought ideas about the contemporary periodical table and encouraged multiple generations to proceed with the study of elements’ properties and interactions.

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This Story also Contains

1. Mendeleev's Periodic Table: Pioneer to the arrangement of elements.
2. Correction of doubtful atomic weights:
3. Defects of Mendeleev's Periodic Table:
4. Lother Meyer:
5. Solved Examples Based On Mendeleev's Periodic Table
6. Conclusion

In this article, we will cover the concept of Mendeleev's Periodic Table observations including the conclusion based on the experiment. This concept falls under the broader category of Classification of Elements and Periodic Table, which is a crucial chapter in Class 11 chemistry. It is not only essential for board exams but also for competitive exams like the Joint Entrance Examination (JEE Main), National Eligibility Entrance Test (NEET), and other entrance exams such as SRMJEE, BITSAT, WBJEE, BCECE, and more. Over the last ten years of the JEE exam (from 2013 to 2023), one question has been asked on this concept, in 2021 from this concept.

Mendeleev's Periodic Table: Pioneer to the arrangement of elements.

Dmitri Mendeleev:

Mendeleev proposed the Periodic Law and constructed his Periodic Table of Elements. At that time, the structure of atoms was unknown and Mendeleev’s idea to consider that the properties of the elements were in some way related to their atomic masses was a very imaginative one. To place certain elements into the correct group from the point of view of their chemical properties, Mendeleev reversed the order of some elements as they did not fit the scheme classification if the order of atomic weights had strictly followed. Mendeleev also had the foresight to leave gaps in the Periodic Table for elements unknown at that time and predict their properties from the trends that he observed among the properties of related elements. Mendeleev’s predictions were proved to be astonishingly correct when these elements were discovered later. Mendeleev’s Periodic Law spurred several areas of research during the subsequent decades.

Mendeleev's Periodic Table:

1. Mendeleev's periodic law: The physical and chemical properties of elements are the periodic function of their atomic weight

2. Characteristic of Mendeleev's periodic table :

   • It is based on atomic weight

   • 63 elements were known, but noble gases were not discovered.

   • He was the first scientist to classify the elements in a systematic manner i.e. in horizontal rows and vertical columns.

   • Horizontal rows are called periods and there were 7 periods in Mendeleev's Periodic table.

   • Vertical columns are called groups and there were 8 groups in Mendeleev's Periodic table.

   • Each group up to VIIth is divided into A & B subgroups.'A' sub-group elements are called normal elements and 'B' sub-group elements are called transition elements.

   • The VIIIth group consisted of 9 elements in three rows (Transitional metals group).

   • The elements belonging to the same group exhibit similar properties.

3. Merits or Advantages of Mendeleev's periodic table :

1. Study of elements: First time all known elements were classified according to their similar properties. So study of the properties of elements becomes easier.

2. Prediction of new elements: It encouraged the discovery of new elements as some gaps were left in it.

   Sc (Scandium) Ga (Gallium) Ge (Germanium) Tc (Technetium)

These were the elements for whom position and properties were well defined by Mendeleev even before their discoveries and he left the blank spaces for them in his table.

Ex. Blank space at atomic weight 72 in the silicon group was called Eka silicon (which means properties like silicon) and the element discovered later was named Germanium.

Similarly, other elements discovered after Mendeleev's periodic table were.

Eka aluminium – Gallium(Ga)

Eka Boron – Scandium (Sc)

Eka Silicon – Germanium (Ge)

Eka Manganese – Technetium (Tc)

Correction of doubtful atomic weights:

Correction was done in the atomic weight of some elements.

Atomic weight = Valency × Equivalent weight.

Initially, it was found that the equivalent weight of Be is 4.5 and it is trivalent (V = 3), so the weight of Be was 13.5 and there is no space in Mendeleev's table for this element. So, after correction, it was found that Be is divalent (V = 2). So, the weight of Be became 2 × 4.5 = 9 and there was a space between Li and B for this element in Mendeleev's table.

– Corrections were done in the atomic weight of elements are – U, Be, In, Au,

Defects of Mendeleev's Periodic Table:

1. The position of hydrogen is uncertain. It has been placed in the lA and VII-A groups because of its resemblance with both groups.

2. No separate positions were given to isotopes.

3. It is not clear whether the lanthanides and actinides are related to IIA or IIB group.

4. Although there is no resemblance except the valency of subgroups A and B, they have been put in the same group.

5. The order of increasing atomic weights is not strictly followed in the arrangement of elements in the periodic table. E.g. – Co (At. wt. 58.9) is placed before I (127) and Ar (39.9) before K (39).

Lother Meyer:

1. He plotted a curve between the atomic weight and the atomic volume of different elements.

2. The following observations can be made from the curve –

   1. Most electropositive elements i.e. alkali metals (Li, Na, K, Rb, Cs, etc.) occupy the peak positions on the curve.

   2. Less electropositive i.e. alkali earth metals (Be, Mg, Ca, Sr, Ba) occupy the descending position on the curve.

   3. Metalloids (B, Si, As, Te, At, etc.) and transition metals occupy the bottom part of the curve.

   4. Most electronegative i.e. halogens (F, Cl, Br, I) occupy the ascending position on the curve.

Note: Elements having similar properties occupy similar positions on the curve.

Based on this curve Lother Meyer proposed that the physical properties of the elements are periodic functions of their atomic wt. and this becomes the base of Mendeleev's periodic table.

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Solved Examples Based On Mendeleev's Periodic Table

Example 1: Eka-aluminium and Eka-silicon are respectively known as:

1) Aluminium and silicon

2) Silicon and aluminium

3) (correct)Gallium and germanium

4) Gallium and Tin

Solution:

According to Mendeleev’s periodic table, Eka-aluminium is known as gallium and Eka-silicon is known as germanium.

Hence, the answer is the option (3).

Example 2: Mendeleev’s periodic table is based on:

1) Atomic number

2) (correct) Atomic weight

3) Ionization enthalpy

4) None of the above

Solution

Mendeleev arranged the elements in horizontal rows and vertical columns in his table in order of their increasing atomic weights. In this way, elements with similar properties occupied the same vertical column.

Hence, the answer is the option (2).

Example 3: Which of the following statements is not correct about Lother Meyer’s curve?

1) The alkali metals occupy the maxima of the curve

2) The transition metals occupy the minimum of the curve

3) (correct) The halogens occupy positions on the descending portions of the curve

4) The alkali earth metals occupy mid positions on the descending portions of the curve

Solution: The Lother Meyer curve. Thus, halogens occupy positions on the ascending portions of the curve.

Therefore, Option(3) is correct

Conclusion

Thanks to the great genius of one of the most outstanding chemists in history, mankind has acquired a unique publication, the Periodic Table of Mendeleev. His way of organizing the elements might be great because not only offered a system of knowing the properties of the elements but also opened doors for further interpretations and breakthroughs in chemistry. In the way that he guessed at and calculated the probable location and characteristics of still undiscovered elements, Mendeleev proved anew the fruitfulness of the mental exercise of rational processes complemented by the observation of the facts. Nevertheless, the further elaboration and modifications, which have been made to the Periodic Table by other chemists, people remember the genius of Mendeleev’s work, which played a crucial role in the formation of contemporary chemistry.