Mandeelev and Modern Periodic Table

Mandeelev Periodic Table

Russian chemist Dmitri Mendeleev studied the physical and chemical properties of 63 known elements and their compounds. After the study, he arranged all the known elements on the basis of increasing atomic weights. As a result, he found that elements with similar properties occur at regular intervals. On the basis of the above observation, Mandeelev formulated the periodic law in 1869 AD. According to Mendeelev's periodic law, " The physical and chemical properties of elements are a periodic function of their atomic weights. " Periodic function means, if elements are arranged in the order of increasing atomic weights, the properties of the elements go on changing with their atomic weights. But after a certain interval, they repeat the properties of the previous elements and fall one below another in the same vertical column. With the help of this, Mendeelev arranged elements according to the increasing atomic weights. Hence, he developed a table known as mandeelev periodic table. In Mandeelev periodic table, elements are classified into horizontal rows called columns and vertical columns called periods. Elements with similar properties are in the same group and elements with gradual changes in properties are in the same period. In Mandeelev periodic table, the 4, 5 and 6 are divided into two halves to place more elements. Half of the elements are placed in the upper left corner and other half in the lower right corner of the box. 

Many elements were not discovered when Mandeelev formed the periodic table. So, he left some gaps for the undiscovered elements and proposed names for them, e.g. Eka-Boron, Eka-Aluminum, Eka-Silicon, Eka-Manganese etc. 

Merits of Mandeelev periodic table 

1. Mandeelev periodic table was the first scientific and systematic study of the all known 63 elements. 
2. In Mandeelev periodic table, there were many gaps for undiscovered elements. After their discovery, they got their proper place in the table. 
3. Mandeelev periodic table helped to correct the doubtful atomic weights of some elements on the basis of their positions in the periodic table, e.g. gold (Au), platinum (Pt), uranium (U) etc. 
4. With the help of Mandeelev period table, properties of several elements were studied correctly. 

Demerits of Mandeelev Periodic Table 

1. Mandeelev could not arrange hydrogen properly because it shows the properties of both alkali metals and halogens. 
2. Mandeelev could not arrange isotopes because they have different atomic weights but same atomic number. 
3. Mandeelev could not arrange Lanthanides and Actinides. 
4. In Mandeelev periodic table, some elements do not obey the periodic law. In some places, elements having more atomic weights were placed before those of less atomic weight. 
5. In Mandeelev periodic table, more reactive alkali metals like Li, Na, K, etc. and less reactive coinage metals (Cu, Ag, Au) etc. were placed together in the same group. 
6. The eight group of Mandeelev periodic table was not divided into sub-groups.
7. Inert gases or noble gases were absent. 

Modern Periodic Table 

A group of chemists led by Henry Moseley studied the physical and chemical properties of all known elements. They found that properties of elements depended more correctly on the atomic number than on atomic weight. Moseley said that atomic number is the fundamental property of the atom. Afterward, they proposed a new law called the modern periodic law. Modern periodic law states that, "Physical and chemical properties of the elements are a periodic function of their atomic numbers. " After the discovery of the modern periodic law, elements were arranged on the basis of increasing atomic numbers. As a result, they found that elements having similar properties fall one after another in the same vertical column called group. Elements having gradual change in the properties were in the horizontal row called period. Hence, a more correct table than Mandeelev table was obtained, which is known as the modern periodic table. So, the table which is obtained after arranging elements on the basis of increasing atomic numbers is called the Modern Periodic Table.

Correction of Defects of Mandeelev Periodic Table by Modern Periodic Table 

In the Modern Periodic Table, elements have been arranged on the basis of increasing atomic numbers. As a result, all the defects of Mandeelev periodic table were removed. The defects and their reasonable corrections are given below: 

1. Position of Hydrogen: Since hydrogen has the least atomic number, i.e. 1, it is kept in group 1 of the modern periodic table, but its position is still controversial due to its ducal characteristics, i.e., characteristics of group 1 and group 17. 

2. Position of Isotopes: Isotopes are elements having the same atomic numbers different atomic weights. So, without any doubt, all isotopes of one elements are kept in one place. 

3. Position of Lanthanides and Actinides: Elements of Lanthanides and Actinides series are kept below the main block of the modern periodic table as they have different properties from the other elements.

4. Correction of periodic law: Some elements do not obey Mandeelev periodic law, but when they are arranged according to atomic number they obey the modern periodic table which is corrected by Moseley. 

5. Position of Alkali metals and Coinage metals: In the modern periodic table, alkali metals (Li, Na, K, etc.) are kept in group IA and coinage metals (Cu, Ag, Au) are in group IB.