In 1869, the Russian chemist Dmitri Ivanovich Mendeleev published his periodic table, marking the most significant breakthrough in the systematic classification of elements. Unlike his predecessors who created static lists, Mendeleev organized the 63 known elements based on their atomic masses and chemical properties, establishing a framework that predicted the existence of elements yet to be discovered.
The Periodic Law
The foundational principle of Mendeleev’s classification is the Periodic Law, which states: The physical and chemical properties of elements are a periodic function of their atomic masses. This means that when elements are arranged in the order of increasing atomic masses, elements with similar physical and chemical properties reappear at regular, predictable intervals.
Structural Features of the Table
Mendeleev organized the elements into a grid composed of horizontal rows and vertical columns.
Periods
The horizontal rows in the periodic table are called periods. Mendeleev’s original table contained a total of twelve rows, which were later consolidated into seven periods as structural understanding evolved.
Groups
The vertical columns are called groups. There are eight groups, designated by Roman numerals from I to VIII.
- Groups I to VII are further divided into two subgroups each, termed Subgroup A (normal or representative elements) and Subgroup B (transition elements).
- Group VIII contains transition elements arranged in triads (such as Iron, Cobalt, and Nickel) and does not have subgroups.
- Noble gases were not part of the original 1869 table as they had not been discovered yet; they were later added as “Group 0” without disturbing the existing order.
Basis of Chemical Classification
To determine placement within groups, Mendeleev focused heavily on the formulae of the hydrides and oxides formed by the elements. For instance, elements in Group I form oxides with the general formula R2O and hydrides with the formula RH (where R represents the element).
Merits and Achievements of Mendeleev’s Classification
Mendeleev’s table was widely accepted because of its remarkable predictive power and scientific flexibility.
Prediction of Undiscovered Elements
Mendeleev left deliberate gaps in his periodic table for elements that were yet to be discovered. Instead of viewing these gaps as flaws, he used the properties of neighboring elements to predict the atomic mass and chemical traits of these unknown elements with astonishing accuracy. He named them by prefixing the Sanskrit word Eka (meaning “one”) to the name of the preceding element in the same group.
| Mendeleev’s Predicted Element | Later Discovered Element |
|---|---|
| Eka-Boron | Scandium (Sc) |
| Eka-Aluminum | Gallium (Ga) |
| Eka-Silicon | Germanium (Ge) |
Correction of Doubtful Atomic Masses
Mendeleev corrected the erroneous atomic masses of several elements by reassessing their position based on chemical valency.
- Example: The atomic mass of Beryllium (Be) was previously thought to be 13.5 based on a mistaken trivalent assumption (valency of 3). Mendeleev realized its properties aligned with Group II (divalent). Applying the formula Atomic Mass=Equivalent Mass×Valency (4.5×2), he corrected its atomic mass to 9.0.
- Similar corrections were made for Indium (In), Gold (Au), and Platinum (Pt).
Accommodation of Noble Gases
When noble gases like Helium (He), Neon (Ne), and Argon (Ar) were discovered in the late 19th century, they could be placed in a completely new group (Group 0) at the edge of the table without altering or disrupting the existing arrangement of elements.
Limitations and Anomalies
Despite its brilliance, the reliance on atomic mass as the fundamental organizing criterion led to several inherent contradictions.
Anomalous Pairs (Inversion of Atomic Mass Order)
In a few instances, Mendeleev placed an element with a slightly higher atomic mass before an element with a lower atomic mass just to maintain the homogeneity of chemical properties within a group.
- Tellurium (Te, mass 127.6) was placed before Iodine (I, mass 126.9) because Iodine shared properties with the halogens in Group VII.
- Cobalt (Co, mass 58.9) was placed before Nickel (Ni, mass 58.7).
- Argon (Ar, mass 39.9) was placed before Potassium (K, mass 39.1) in later versions.
Position of Isotopes
Isotopes are atoms of the same element with identical chemical properties but different atomic masses. According to Mendeleev’s law, isotopes (like Protium 1H, Deuterium 2H, and Tritium 3H) should occupy different slots in the table. However, giving them separate positions would break the group uniformity based on chemical traits, a dilemma Mendeleev could not resolve.
Anomalous Position of Hydrogen
Hydrogen exhibits properties of both Alkali Metals (Group I) by forming unipositive ions and Halogens (Group VII) by forming diatomic molecules and combining with metals to form covalent compounds. Mendeleev could not assign a singular, logically flawless position to Hydrogen.
Grouping of Dissimilar Elements and Separation of Similar Elements
- Coinage metals like Copper (Cu), Silver (Ag), and Gold (Au) were placed in Group IB alongside highly reactive alkali metals like Sodium (Na) and Potassium (K) in Group IA, despite having vastly different reactivities.
- Chemically similar elements like Platinum (Pt) and Gold (Au) were placed in completely separate groups.
UPSC Prelims Core Insights
- Transition from Mass to Number: Mendeleev’s anomalies occurred because atomic mass is not the fundamental property governing chemical identity. This was corrected in 1913 by Henry Moseley, who showed that atomic number (Z) is the true basis of periodic classification, leading to the Modern Periodic Table.
- Sanskrit Nomenclature Trivia: Mendeleev used the Sanskrit prefix Eka- out of respect for the ancient Indian grammarian Pāṇini, whose structured approach to the Sanskrit language mirrored the structural grids of the periodic table.