Atomic Models and Structure
🔍 Discovery of Subatomic Particles
Scientists found that atoms contain smaller particles:
- Protons (+ charge) were discovered in 1919 as the smallest positive ions from hydrogen gas.
- Neutrons (no charge) were found by Chadwick in 1932 by hitting beryllium with α-particles. They’re slightly heavier than protons.
| Particle | Symbol | Charge (C) | Mass (kg) |
|---|---|---|---|
| Electron | e | -1.602 × 10-19 | 9.109 × 10-31 |
| Proton | p | +1.602 × 10-19 | 1.673 × 10-27 |
| Neutron | n | 0 | 1.675 × 10-27 |
⚡ Millikan’s Oil Drop Experiment
How we measured electron charge:
- Oil droplets were sprayed between charged plates.
- X-rays gave droplets charges by ionizing air.
- By watching how fast/slow they fell in electric fields, Millikan found:
Charge always comes in multiples: \( q = n \cdot e \) (\( n = 1, 2, 3… \))
🎯 Key insight: Charge is quantized (comes in fixed packets)!
🍰 Thomson’s “Plum Pudding” Model (1898)
- Atom = sphere of positive charge with electrons stuck in it (like raisins in a pudding).
- Mass spread evenly through the atom.
- Couldn’t explain later experiments ❌
☀️ Rutherford’s Nuclear Model (1911)
Experiment: Shot α-particles at thin gold foil:
- 💨 Most passed straight through → atom is mostly empty space.
- 🔄 Some deflected at small angles → positive charge is concentrated.
- 💥 Very few bounced back (1 in 20,000!) → positive part is tiny but heavy.
Conclusions:
- Atom has a dense, positive nucleus (size: 10-15 m vs. atom’s 10-10 m).
- Electrons orbit nucleus like planets around the sun 🌎→☀️.
- Opposite charges hold atom together.
🔢 Atomic Number & Mass Number
- Atomic number (Z) = protons (or electrons in neutral atoms).
- Mass number (A) = protons + neutrons.
- Symbol notation: \( ^A_ZX \) (e.g., \( ^{35}_{17}Cl \))
🌀 Isotopes vs. Isobars
- Isotopes: Same Z (element), different A (neutrons).
Example: \( ^{12}_6C \), \( ^{13}_6C \), \( ^{14}_6C \) - Isobars: Same A, different Z (different elements).
Example: \( ^{14}_6C \) and \( ^{14}_7N \)
🧪 Fun fact: Isotopes act similarly in chemistry because chemical properties depend on electrons!
⚠️ Problems with Rutherford’s Model
- Electrons moving in orbits should lose energy as radiation → spiral into nucleus in 10-8 seconds! 🌪️
- But atoms don’t collapse → model can’t explain stability 😕
- Also silent on electron arrangement and energies.
✨ Toward Bohr’s Model
Two clues helped fix Rutherford’s model:
- Light has dual nature (wave + particle).
- Atomic spectra (unique “light fingerprints” of elements).
Electromagnetic radiation: Produced by accelerating charged particles. Travels as waves with electric + magnetic fields (Fig 2.6).
🚀 Important for NEET!
- Rutherford’s α-scattering: Conclusions about nuclear size & atom structure.
- Proton/neutron discovery: Experiments (canal rays, Chadwick) & properties.
- Isotopes/Isobars: Definitions, examples, and notation (\( ^A_ZX \)).
- Rutherford model flaws: Why orbiting electrons should collapse (classical physics vs. reality).
- Millikan experiment: How charge quantization was proven (\( q = n \cdot e \)).
Keep these in mind — they’re NEET favorites! 💯
