Mendeleev Periodic Table was introduced in 1869, following the rejection of Newlands Octave Law. Elements were arranged in Mendeleev’s periodic chart according to their fundamental property, atomic mass, and chemical characteristics.

At present, we know 118118 elements. These elements are placed in 1818 groups and 77 periods in the Modern Periodic Table. To form the Modern Periodic Table, the Great Scientist Dmitri Ivanovich Mendeleev gave a clear basic idea. In this article, you will explore Mendeleev’s Periodic Table Law and its properties, merits, and demerits in the simplest way.

What is the Basis of Mendeleev’s Periodic Table?

Mendeleev arranged all the \(63\) elements known at that time in the form of a table known as Mendeleev’s periodic table. In Mendeleev’s periodic table, the elements were arranged horizontally in \(10\) series in the order of their increasing atomic weights. These series were divided into \(7\) horizontal columns (period) and \(8\) vertical columns (groups).

Who is Called the Father of the Periodic Table?

Dmitri Ivanovich Mendeleev is called the Father of the Periodic Table. He was born in Tobolsk (Siberia) on \(8\) February \(1834.\) After his early education, he joined University. He received a Master’s degree in chemistry in 1856 and a doctoral degree in \(1865.\) He was appointed professor of general chemistry in \(1867\) at the University of St. Petersburg.

In \(1869,\) he proposed the famous Periodic law and constructed the periodic table of elements. In \(1890\) he resigned his professorship and was appointed as director of the Bureau of weights and measures. Element with atomic number \(101\) is named mendelevium by American scientist Glenn T Seaborg, the discoverer of the element. He died in February \(1907.\)

What is Mendeleev’s Periodic Law?

The first meaningful and remarkable contribution in the field of classification of elements was made by the Russian chemist Dmitri I. Mendeleev in \(1869.\) Based on his studies, he developed a law known as Mendeleev’s Periodic Law. This law states that “The physical and chemical properties of the elements are the periodic function of their atomic weights”.

The following two inferences are drawn from his periodic law:

1. Atomic masses of elements determine their physical and chemical properties.
2. Elements with similar physical and chemical properties are arranged together in the increasing atomic mass.

Properties

Mendeleev placed the elements having similar properties in the same group in the increasing order of atomic masses. In several places, he ignored the order of atomic masses to group together the elements having similar properties.

With the discovery of more and more elements, Mendeleev’s periodic table was revised, and the new elements were included in the appropriate places. This table is called the modified form of Mendeleev’s periodic table.

The main characteristics of Mendeleev’s Periodic Table are:

1. In this table, the elements are arranged in vertical columns called groups and horizontal rows known as periods.
2. There are \(9\) groups indicated by Roman numbers as I, II, III, IV, V, VI, VII, VIII, and zero. The elements belonging to the first \(7\) groups have been divided into subgroups designated as A and B based on similarity in properties. The elements that are placed on the left-hand side of each group constitute subgroup A, while those on the right-hand side form subgroup B. Group VIII consists of \(9\) elements which are arranged in triads. The zero group contains elements belonging to inert (noble) gases, and the elements present have zero valencies.
3. There are seven periods (numbered from \(1\) to \(7\)). To accommodate more elements, periods \(4, 5, 6,\) and \(7\) are divided into two halves. The first half of the elements are placed in the upper left corner, and the second occupies the lower right corner in each box.
4. The first period with only two elements is called the shortest period. The second and \({\rm{3rd}}\) periods with \(8\) elements each are known as short periods. Periods categorized based on \({\rm{4th}}\) and \({\rm{5th}}\) have \(8\) elements each and are designated as long periods. The \({\rm{6th}}\) period with \(32\) elements is regarded as the longest period. The last period, i.e., The \({\rm{7th}}\) period, is not complete since the elements are being added in this period.

Advantages of Mendeleev’s Periodic Table

1. Systematic study of Chemistry: Mendeleev’s periodic table categorised the elements for the first time in a symmetric way. By knowing the properties of one element, the properties of other elements and their compounds in the group are guessed. This made the study of elements easier.

2. Prediction of New Elements: At the time when Mendeleev’s periodic table was formulated, only \(63\) elements were known. Therefore, while arranging the elements according to their properties, Mendeleev’s left some blank places (gaps). These gaps represented unknown elements. Further, Mendeleev predicted the properties of these unknown elements on the basis of their position.

For example, gallium and germanium were not known at the time of Mendeleev. Mendeleev named these elements Eka-aluminium and Eka-silicon because he believed that they would resemble aluminium and silicon, respectively, in their properties. The properties of Eka-aluminium discovered and named as gallium by De Baisbaudron, and the properties of Eka-silicon, discovered and named as germanium by Winkler is corresponds to Properties predicted by Mendeleev.

Comparison of the properties of Eka-Aluminium and Eka-Silicon as predicted by Mendeleev and those found experimentally.

Property	Eka-Aluminium \(\left( {\rm{M}} \right)\) (predicted)	Gallium \(\left( {{\rm{Ga}}} \right)\) (found)	Eka-silicon \(\left( {\rm{M}} \right)\) (predicted)	Germanium \(\left( {{\rm{Ge}}} \right)\) (found)
Atomic mass	\(68\)	\(70\)	\(72\)	\(72.6\)
Density \(\left( {{\rm{g/c}}{{\rm{m}}^{\rm{3}}}} \right)\)	\(5.9\)	\(5.94\)	\(5.5\)	\(5.36\)
Melting point \(\left( {\rm{K}} \right)\)	Low	\(302.78\)	High	\(1231\)
Formula of oxide	\({{\rm{M}}_2}{{\rm{O}}_3}\)	\({\rm{G}}{{\rm{a}}_2}{{\rm{O}}_3}\)	\({\rm{M}}{{\rm{O}}_{\rm{2}}}\)	\({\rm{Ge}}{{\rm{O}}_{\rm{2}}}\)
Formula of chloride	\({\rm{MC}}{{\rm{l}}_{\rm{3}}}\)	\({\rm{GaC}}{{\rm{l}}_{\rm{3}}}\)	\({\rm{MC}}{{\rm{l}}_{\rm{4}}}\)	\({\rm{GeC}}{{\rm{l}}_{\rm{4}}}\)

3. Correction of Atomic Masses: Correct atomic masses of many elements was determined with the use of Mendeleev’s periodic table.

For example, the atomic mass of beryllium was given as \(14.8\) by Dulong and Petit’s law. But Mendeleev gave the correct atomic mass of beryllium as \(9,\) based on the position of beryllium. Since beryllium is bivalent based on its position in Mendeleev’s periodic table. The atomic mass of beryllium \({\rm{ =  Equivalent  mass }} \times {\rm{Valency  =  4}}.{\rm{5}} \times {\rm{2  =  9}}\)

Defects of Mendeleev’s Periodic Table

1. Position of hydrogen: Based on properties, hydrogen should have been placed in both group IA and group VIIA. Since hydrogen exhibits both the properties of alkali metals (IA group) and halogens (VIIA group). But, hydrogen has been placed in the IA group in this periodic table.

2. Position of isotopes: In Mendeleev’s periodic table, elements are arranged in the increasing atomic masses. Therefore, isotopes of an element should be placed at different places. But this will be inconsistent because isotopes possess similar chemical properties.
Example: Protium, deuterium, and tritium with atomic weights \(1,\) \(2,\) and \(3\) respectively should be placed at different places. However, these have not been given in different places in the periodic table.

3. Position of lanthanides and actinides: The \(14\) elements following lanthanum atomic number \(58\) to \(71\) are lanthanides and \(14\) elements following actinium, atomic number \(90-103\) are actinides. These arrangements are anomalous in Mendeleev’s periodic table since they are not following Mendeleev’s periodic law.

4. Elements with variable valency: A large number of elements show more than one valency. But this was not mentioned in Mendeleev’s periodic table.

5. Anomalous pair of elements: Several elements are not arranged in the increasing atomic mass, i.e., not placed according to Mendeleev’s periodic law.
For example, argon (atomic number \( = 39.9\)) is placed before potassium (atomic number \(= 39.1\)). Similarly, tellurium (atomic mass \(= 127.61\)) is placed before Iodine (atomic mass \(= 126.91\)). If the criterion of atomic weight is strictly followed, the potassium should be placed before the argon. That is, potassium should be placed in the zero group while argon in the IA group.

6. Similar elements are not placed in the same groups: The elements like silver and thallium, barium and lead, copper and mercury showing similar properties are placed in a different group in Mendeleev’s periodic table.

7. Dissimilar elements placed in the same groups: The elements in the subgroup A and B of a particular group show different physical and chemical properties but still they are placed in the same group in Mendeleev’s periodic table. Eighth group: This group is full of anomalies. Except for osmium, no other element of this group shows the group valency, i.e., no other element is octavalent. The atomic weight and several other properties are also anomalous.

8. Cause of periodicity: no proper explanation has been offered for the fact why the elements placed in a group show resemblance in their properties.

The defects in Mendeleev’s Periodic Table were overcome with the Modern Periodic table. This table is based on the Modern periodic Law given by Henry Moseley. Modern Periodic Law states that “Physical and chemical properties of the elements are the periodic function of their atomic number”.

Summary

Mendeleev arranged all the \(63\) elements known at that time in the form of a table known as Mendeleev’s periodic table. In Mendeleev’s periodic table, the elements were arranged horizontally in \(10\) series in the order of their increasing atomic weights.

This article enlightened the great contribution of Mendeleev in the arrangement of elements in Mendeleev’s Periodic Table, its properties, merits, and demerits. We studied in detail about Mendeleev, his contribution to arranging all the elements and how he gave the famous periodic table that we use to date. Then we had some frequently asked questions on this topic.

FAQs

Q.1. What is the Law of Mendeleev’s Periodic table?
Law of Mendeleev’s Periodic table states that “The physical and chemical properties of the elements are the periodic function of their atomic weights”.

Q.2. Why did Mendeleev leave gaps in the Periodic table?
Ans: Mendeleev left gaps in the Periodic table for the undiscovered elements like scandium, germanium, gallium, etc. 

Q.3. How did Mendeleev arrange the Periodic table?
Ans: Mendeleev arranged elements in the increasing order of their atomic mass in the Periodic table.

Q.4. How many elements does Mendeleev’s Periodic table have?
Ans: The number of elements in Mendeleev’s Periodic table is \(63\). 

Q.5. What was the problem with Mendeleev’s Periodic table?
Ans: The few problems of Mendeleev’s Periodic table are it could give the correct position for hydrogen, isotopes of elements, variable valency of the elements. He placed dissimilar elements in the same group, and the reason for the periodicity was not explained.

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