🔥 Dalton’s Atomic Theory (1808)
John Dalton proposed these fundamental ideas about atoms:
- 💧 Matter is made of atoms – tiny, indivisible particles
- ⚖️ All atoms of an element are identical – same mass & properties
- 🧪 Compounds form when atoms of different elements combine in fixed ratios
- 🔄 Chemical reactions rearrange atoms – no creation/destruction of atoms
✅ Success: Explained laws of chemical combination
❌ Limitation: Couldn’t explain why gases combine in simple volume ratios (like 2L H₂ + 1L O₂ → 2L water vapor)
⚖️ Atomic Mass
Atoms are super tiny! We measure their mass using atomic mass units (amu):
- Modern standard: Carbon-12 atom \( \left( ^{12}\text{C} \right) \)
- 1 amu = exactly \( \frac{1}{12} \)th mass of one \( ^{12}\text{C} \) atom
- \( ^{12}\text{C} \) is assigned a mass of exactly 12 amu
Fun fact: Earlier, hydrogen was used as reference (mass = 1), but carbon-12 became standard in 1961 ✨
🧪 Important History
- Avogadro (1811) suggested gases combine in volume ratios because they’re made of 2-atom molecules (like H₂, O₂)
- His ideas gained recognition in 1860 at a chemistry conference in Germany 🌍
🚀 NEET Super Focus
Must-Know Concepts:
- Dalton’s 4 postulates (especially fixed ratios & atom indivisibility)
- Why Dalton’s theory failed to explain gas volume combinations
- Definition of 1 amu = \( \frac{1}{12} \times \) mass of \( ^{12}\text{C} \) atom
- Carbon-12 as the modern atomic mass standard
- Avogadro’s role in explaining gas reactions
💡 Remember
Dalton called atoms “indivisible” (meaning unbreakable), but we now know they’re made of smaller particles! His theory was revolutionary for chemistry 🎉
