Genesis of the Periodic Table
🔍 Early Attempts at Classification
- Dobereiner’s Triads (1829): Grouped elements in sets of 3 (e.g., Li/Na/K, Cl/Br/I).
- ⚖️ Middle element’s atomic weight ≈ average of the other two
- ✨ Properties of middle element were intermediate
Example: Cl (35.5), Br (80), I (127) → Br’s atomic weight = (35.5 + 127)/2 ≈ 81.25 (close to 80)
- Newlands’ Law of Octaves (1865): Arranged elements by atomic weight and noticed every 8th element had similar properties (like musical notes 🎵).
Example: Starting from Li (1st), Na (8th) had similar properties.
⚠️ Only worked for elements up to calcium (Ca).
🌟 The Breakthrough: Mendeleev & Meyer (1869)
- Both scientists independently proposed: “Properties repeat periodically when elements are arranged by increasing atomic weight” 🔄
- Meyer plotted physical properties (atomic volume, melting point) vs. atomic weight → saw repeating patterns 📈
- Mendeleev created the first practical periodic table:
- Vertical columns = groups (elements with similar properties)
- Horizontal rows = periods
- 🧠 Left gaps for undiscovered elements and predicted their properties!
🔮 Mendeleev’s Genius Predictions
He swapped elements when properties didn’t match atomic weight order (e.g., put Iodine after Tellurium despite lower atomic weight). Predicted:
| Element | Predicted Property | Actual Discovery |
|---|---|---|
| Eka-Aluminium (predicted) | Atomic weight: 68 Density: 5.9 g/cm³ Oxide: \(\text{E}_2\text{O}_3\) | Gallium (Ga) Atomic weight: 70 Density: 5.94 g/cm³ Oxide: \(\text{Ga}_2\text{O}_3\) |
| Eka-Silicon (predicted) | Atomic weight: 72 Density: 5.5 g/cm³ Oxide: \(\text{EO}_2\) | Germanium (Ge) Atomic weight: 72.6 Density: 5.36 g/cm³ Oxide: \(\text{GeO}_2\) |
📜 Mendeleev’s Periodic Law
“The properties of elements are a periodic function of their atomic weights.”
🎯 NEET Must-Know Concepts
- Dobereiner’s Triads: Concept of 3-element groups with atomic weight patterns.
- Newlands’ Law of Octaves 🎵: Early periodicity model (every 8th element).
- Mendeleev’s Table: How he arranged elements and left gaps for undiscovered ones.
- Mendeleev’s Predictions 🔮: Accuracy for Gallium/Eka-Al and Germanium/Eka-Si.
- Periodic Law Statement: “Properties = periodic function of atomic weights.”
💡 Remember: Mendeleev prioritized chemical properties over atomic weights when arranging elements – that’s why his table worked so well!
