Mendeleev's Periodic Table
Dmitri Mendeleev (1869) laid the foundation of the modern classification of elements. He arranged the 63 elements known at that time in increasing order of their Atomic Masses.
1. Mendeleev's Periodic Law
"The physical and chemical properties of elements are a periodic function of their atomic masses."
2. Structure of the Table
Mendeleev's original table was divided into:
- Groups: 8 Vertical columns (I to VIII). Groups I-VII were subdivided into subgroups A and B. Group VIII contained transition metals in sets of three.
- Periods: 6 Horizontal rows.
3. Merits of Mendeleev's Table
A. Prediction of New Elements
Mendeleev left gaps for elements that were not yet discovered and boldly predicted their properties. He named them by prefixing Eka (Sanskrit for 'one') to the name of the preceding element in the same group.
| Predicted Name | Discovered Element | Atomic Mass (Approx) |
|---|---|---|
| Eka-Boron | Scandium ($Sc$) | 44 |
| Eka-Aluminium | Gallium ($Ga$) | 68 |
| Eka-Silicon | Germanium ($Ge$) | 72 |
| Eka-Manganese | Technetium ($Tc$) | 100 |
B. Correction of Atomic Masses
He corrected the atomic masses of several elements (e.g., Beryllium, Indium, Gold, Platinum) based on their valency and position.
Previous Mass = $13.5$ (Valency 3)
Corrected Mass = $9.01$ (Valency 2)
4. Demerits (Limitations)
Key Failures
- Position of Hydrogen: Hydrogen resembles both Alkali Metals (Group I) and Halogens (Group VII). Its position was anomalous.
- Position of Isotopes: Isotopes have different atomic masses but same chemical properties. There was no place for them.
- Anomalous Pairs: To fit elements with similar properties together, he sometimes inverted the order of atomic mass (Placing heavier element before lighter one).
- Argon ($39.9$) before Potassium ($39.1$)
- Cobalt ($58.9$) before Nickel ($58.7$)
- Tellurium ($127.6$) before Iodine ($126.9$)
- Grouping of Dissimilar Elements: Chemically different elements like Coinage metals ($Cu, Ag, Au$) were grouped with Alkali metals ($Li, Na, K$).
5. Transition to Modern Table
The discovery of atomic number by Henry Moseley resolved these anomalies. The fundamental property is Atomic Number, not Atomic Mass. This led to the Modern Periodic Law.
Mendeleev Quiz
Test your concepts on Periodic Classification. 10 MCQs with explanations.
No comments:
Post a Comment