1.2.1Additional lessons
Nuclear Model of the Atom — IB Chemistry HL Revision Notes & Topical Questions
5 min read 4 parts Lesson 4 of 172
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Part 1 of 4Before the Nuclear Model
Early models of the atom changed as new evidence appeared.
Dalton
- Dalton Atoms were indivisible solid spheres, with all atoms of an element identical.
Thomson
- Thomson The discovery of the electron led to a plum-pudding model: negative electrons embedded in a diffuse positive sphere.
Prediction
- Prediction If the plum-pudding model were right, positively charged alpha particles would pass through with only small deflections.
Testing
- Testing Rutherford's group fired alpha particles at thin gold foil and detected where they ended up.
Part 2 of 4Gold Foil Results
Gold Foil Results
Most pass through
- Most pass through The majority of alpha particles went straight through, so most of the atom is empty space.
Some deflected
- Some deflected A small fraction were deflected through large angles, so there is a concentrated positive charge.
Few rebound
- Few rebound A very small number bounced almost straight back, so the positive charge is in a very small, dense, massive region.
Conclusion
- Conclusion The atom has a tiny, dense, positive nucleus surrounded by electrons in the remaining volume.
Part 3 of 4The Nuclear Model
The model that explains the results has these features.
1
The nucleus contains the positive charge and nearly all the mass of the atom, within a radius about 10⁵ times smaller than the atom.
2
Electrons move in the space outside the nucleus, which makes up almost all of the volume of the atom.
3
The atom is neutral overall because the nuclear positive charge equals the total negative charge of the electrons.
4
Later work showed that electrons occupy fixed energy levels instead of orbiting freely, refining the model.
5
Models are replaced when new evidence cannot be explained by them, which is how scientific knowledge advances.
Part 4 of 4Exam Checklist & Tips
1.2.1 – Key Points for the Exam
- Nucleus The tiny, dense, positively charged centre of an atom.
- Alpha particle A helium nucleus with a charge of +2 emitted in some radioactive decays.
- Plum-pudding model An early model with electrons embedded in a positive sphere.
- Scattering The change in direction of particles after interacting with the atoms in a target.
Common Mistakes
- Saying the results showed that atoms have electrons, which was known before.
- Claiming that most alpha particles were deflected, when most passed straight through.
- Stating that the nucleus takes up most of the volume of the atom.
- Describing the nucleus as negatively charged because alpha particles are repelled.
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