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📘Easy summary
Lavoisier first classified elements as metals and non-metals. In 1829 Döbereiner grouped elements into triads (e.g. Li, Na, K) where the atomic weight of the middle element was roughly the average of the other two. In 1864 John Newlands first observed periodicity: arranging 62 elements by increasing atomic mass, every eighth element resembled the first (law of octaves). Lothar Meyer plotted atomic volume against atomic weight and his curves also showed periodicity. In 1869 Mendeleev, the father of the periodic table, arranged 63 elements by increasing atomic mass, left gaps for undiscovered elements and predicted their properties. In 1913 Moseley showed that atomic number is the basis: properties of elements are periodic functions of their atomic numbers (modern periodic law).
⭐Key points
- Döbereiner: triads (1829).
- Newlands: octaves, first periodicity (1864).
- Mendeleev: atomic mass, gaps, predictions (1869).
- Moseley: atomic number is the basis (1913).
🔬Diagram
🌿Real-life example
Triad Li (7), Na (23), K (39): (7 + 39) ÷ 2 = 23, the atomic mass of Na.
💡Exam tip
- Remember who used atomic mass (Mendeleev) and who used atomic number (Moseley).
✅Quick check
1. The modern periodic law is based on:
2. Every eighth element resembling the first was noted by:
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