Mendeleev’s Periodic Table
In 1871, Dimtri Mendeleev also tried to arrange elements in periodic order. In 19th century, his element arrangement was most successful as compared to early attempts made by other scientists like Dobereiner and Newland.
In this session we will discuss about following topics:
1. Mendeleev’s Periodic Table
2. Mendeleev’s Periodic Table Achievements
3. Mendeleev’s Periodic Table Limitations
In previous sessions, we discussed about the terminologies required for better understanding of periodic table & early attempts made by other scientists:
Mendeleev’s Periodic Table
Dmitri Mendeleev was a chemist and inventor. He is best remembered for formulating the periodic law and creating a far sighted version of periodic table of elements.
Till the time of Mendeleev approx. 63 elements were discovered.
Following were the features for his arrangement of elements:
- Elements were organized in the increasing order of their atomic mass
- Elements were organized in Columns (not Rows)
- Columns of table were numbered from I to VIII
- Each column would contain elements with similar properties
- Columns were called Groups
- Because the elements in a group were similar but not identical to one another
Because of the repeating pattern of element arrangement, Mendeleev’s table of elements was called a Periodic Table.
Achievements – Mendeleev’s Periodic Table
In 19th century, Mendeleev’s periodic table provides the best arrangement of elements. It answers the questions and resolves the limitations left in the early attempts of elements arrangement.
Following are the achievements of Mendeleev’s periodic table:
- Systematic Study of Elements
- Mendeleev made a systematic study of all the elements know at that time and tried to arrange large number of elements into systematic order of Groups and Periods
- Arrangement of Elements in Ascending Order of Atomic mass
- Element properties were also studied, along with increasing order of atomic mass
- Elements were arranged in ascending order of atomic mass with similar properties
- Ex. Cobalt with atomic mass 58.9 was placed before Nickel with atomic mass 58.7
- Prediction of New Elements
- He was able to predict existence of elements that were not discovered that time
- Blank spaces were left in periodic table for those elements
- Correction of Doubtful Atomic Mass
- Atomic masses known of some elements were wrong at that time
- He attempted to correct atomic masses of certain elements with the help of their expected positions and properties
Limitations – Mendeleev’s Periodic Table
Despite being the most successful arrangement of elements in 19th century, there were still some limitations and questions that it was not able to resolve and answer.
Following are its limitations:
- Position of Hydrogen
- Since, properties of Hydrogen resembles both with Alkali Metals & with Halogen
- It was not able assign a correct position to Hydrogen
- Anomalous Position of Isotopes
- Isotopes – An Element with same Atomic Number (Z) but different Atomic Mass (A)
- Element arrangement was based on atomic mass but isotopes were not given any position
- Isotopes would have disturbed the symmetry of the periodic table
- Wrong Order of Atomic Mass
- Element properties and atomic mass both were given importance for the elements arrangement
- But, irregular pattern in increasing order of atomic mass was not explained
- Uncertainty in Discovery of New Elements
- Uncertain to predict how many elements could be discovered between two elements
- Also, if new elements are exists or not
References:
- Learning Course for full lifetime access – Periodic Classification of Elements
- Learning Videos – Periodic Classification of Elements
- Modern Periodic Table
- Newland’s Law of Octaves
- Periods & Groups
- Periods in Detail
- Groups in Detail