Mendeleev’s Periodic table: An Introduction
Mendeleev's periodic table was solely based on atomic mass whereas the modern periodic table relies on atomic numbers. The periodic table is the classified arrangement of chemical elements based on their physical and chemical properties. The modern periodic table we use was created after various alterations to the periodic table originally given by Dmitri Mendeleev. His structure was with columns and rows which is present in the modern periodic table also. The repetition of chemical properties of elements was observed by Mendeleev, hence the term period was used. The columns group together elements with the same qualities, so they are called groups and rows are periods as sets of elements repeat each time due to the similarity of their properties. Mendeleev did not validate isotopes which are atoms of same element but different weights. The modern periodic table is an improvement of the work of many chemists and scientists to create order out of chaos.
What is a Modern Periodic Law?
The Russian chemist 1869 created the basic framework for the modern periodic table, he would rearrange the elements if they did not fit into the group he was putting them into, this was the famous Dmitri Mendeleev, who is also known as the father of the modern periodic table. The modification of Mendeleev's periodic law is today called modern periodic law. The atomic number is equal to the number of electrons or protons in a neutral atom. Periodic law is chemistry's most relevant concept in dealing with chemical elements and their aspects such as electronegativity, atomic radius, or ionising power, it is a huge help to chemists as the elements are arranged in order of their similar chemical and physical properties.
What is a Modern Periodic table?
The modern periodic table is an arrangement of all elements, according to their increasing atomic number and similar chemical properties. They are organised in a tabular arrangement where a row is known as a period and a column is a group.
Elements arranged in the same group will be possessing the same valence electron configuration so they will have the same chemical properties.
Achievements of Mendeleev Periodic Table
Mendeleev kept some blank spaces in the periodic table for the elements which hadn't been discovered. Then he predicted the properties of certain elements before they were even discovered and on their discovery if they were found absolutely correct. Mendeleev's periodic table could fit in the noble gases when they were discovered. He ascertained the formulae of the oxides and hydrides as their basic properties for their classification in the periodic table. Mendeleev's periodic table helped in the correction of the atomic mass of some elements. It contained all known elements in order of increasing atomic mass.
Limitations of the Mendeleev Periodic Table
Besides being a path-breaking discovery, Mendeleev's Periodic table had quite a few demerits:
1. Anomalous pairs: Certain pairs of elements do not follow Mendeleev's principles, such as Cobalt has a higher atomic mass but comes before Nickel. Tellurium comes before Iodine.
2. Isotonic arrangement: According to Mendeleev's periodic law, isotopes had to be given separate places because of their different atomic masses.
3. Hydrogen: It wasn't given a fixed place as it resembles alkali metals and halogens also so that a correct position couldn't be given to it.
Interesting Facts
Mendeleev predicted certain elements but left a gap for them in the periodic table; he gave them hypothetical names such as Eka - aluminium/Eka - silicon.
Mendeleev was awarded the Copley medal, Davy medal and Demidov prize for his contribution.
Key Features
The modern periodic table is an arrangement of all elements according to their increasing atomic number and similar chemical properties, whereas Mendeleev’s Table, it's according to atomic mass.
There are many limitations in Mendeleev’s table that lead to the formation of the modern periodic table.
The elements in the same period of the modern periodic table will possess an increasing order of valence electrons.
The modern periodic table consists of all 118 elements, and maintains uniformity throughout.
FAQs on Making Order Out of Chaos: Mendeleev's Periodic Table
1. What was the fundamental principle behind Mendeleev's periodic table?
The fundamental principle was Mendeleev's Periodic Law, which states that the physical and chemical properties of elements are a periodic function of their atomic masses. He arranged the 63 elements known at the time in order of increasing atomic mass, organising them into a table with vertical columns (groups) and horizontal rows (periods).
2. What were the main achievements of Mendeleev's periodic table?
Mendeleev's periodic table had several key achievements that made it a cornerstone of chemistry:
It systematically organised all known elements, making their study much simpler and more logical.
He left gaps for undiscovered elements and accurately predicted their properties. For example, he predicted 'Eka-aluminium' and 'Eka-silicon', which were later discovered as Gallium and Germanium, respectively, with properties matching his predictions.
His table helped in correcting the atomic masses of several elements, such as Beryllium, whose mass was corrected from 13.5 to 9.
It could accommodate noble gases when they were discovered later by creating a new 'zero' group without disturbing the existing order.
3. How did Mendeleev's decision to leave gaps in his table demonstrate its predictive power?
Leaving gaps was a sign of Mendeleev's remarkable foresight. Instead of viewing them as defects, he boldly proposed that these gaps represented elements yet to be discovered. He even predicted the properties (like atomic mass, oxide formula, and density) of these missing elements based on their expected position in the table. When elements like Gallium (Eka-aluminium) and Germanium (Eka-silicon) were discovered later, their properties matched his predictions almost perfectly, providing powerful evidence for the validity of his periodic law.
4. If elements were arranged by increasing atomic mass, why was cobalt (Co) placed before nickel (Ni)?
This is a classic example of an anomaly in Mendeleev's periodic table. Cobalt has a slightly higher atomic mass (58.9 u) than nickel (58.7 u). However, Mendeleev placed cobalt before nickel because cobalt's properties were more similar to rhodium (Rh) and iridium (Ir) in the same group, while nickel's properties better matched those of palladium (Pd) and platinum (Pt). He prioritised grouping by similar chemical properties over strictly following the atomic mass order, highlighting a limitation of using atomic mass as the sole basis.
5. How did the discovery of isotopes challenge Mendeleev's Periodic Law?
The discovery of isotopes posed a significant challenge. Isotopes are atoms of the same element with the same chemical properties but different atomic masses (e.g., Chlorine-35 and Chlorine-37). According to Mendeleev's law, these isotopes should be given different positions in the table because they have different atomic masses. However, since they have identical chemical properties, they must occupy the same position. This created a contradiction that his 'atomic mass' based system could not resolve, indicating that atomic mass was not the most fundamental property for classification.
6. What is the main difference between Mendeleev's periodic table and the Modern Periodic Table?
The main difference lies in the fundamental property used for classification.
- Mendeleev's Periodic Table arranged elements in order of increasing atomic mass.
- The Modern Periodic Table, based on Henry Moseley's work, arranges elements in order of increasing atomic number (the number of protons).
7. Why couldn't a fixed position be assigned to hydrogen in Mendeleev's periodic table?
Hydrogen's position was a major ambiguity because it exhibited properties similar to two different groups. On one hand, it resembled alkali metals (Group 1) by forming a positive ion (H+) and having a valency of +1. On the other hand, it also resembled halogens (Group 17) by existing as a diatomic molecule (H₂) and forming covalent bonds. Because of this dual nature, Mendeleev could not assign a single, correct position for hydrogen, making it a significant and unresolved limitation of his table.
