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1.2.1Additional lessons

Nuclear Model of the Atom — IB Chemistry SL Revision Notes & Topical Questions

5 min read 4 parts Lesson 4 of 98

📚 IB Chemistry SL Hub / Topical Revision
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Curriculum Equation Key Radioactive Decay & Half-Life FormulasFast Exam Solution
N(t) = N₀ × (1/2)t / t½ | λ = ln(2) / t½ ≈ 0.693 / t½ | E = Δm × c²
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Part 1 of 3Early Models

1.2.1 Nuclear Model of the Atom

Ideas about the atom changed as new evidence appeared.

1
Dalton described atoms as tiny solid spheres, and Thomson later found that they contain negative electrons.
2
Thomson's model pictured a positive sphere with electrons embedded in it like fruit in a pudding.
3
Rutherford's group directed alpha particles at a very thin gold foil and detected where the particles went.
4
The results contradicted the earlier model and led to the nuclear atom with a tiny, dense, charged centre.

Part 2 of 3The Nuclear Atom

The Nuclear Atom
Nucleus
  • Contains the protons and neutrons, is positively charged, and holds nearly all the mass of the atom.
Electrons
  • Move around the nucleus in the large volume surrounding it, held there by electrostatic attraction.
Scale
  • The nucleus is about 10⁻¹⁵ m across and the atom about 10⁻¹⁰ m, so the nucleus is a tiny fraction of the volume.
Neutrality
  • The atom is neutral because the number of electrons equals the number of protons in the nucleus.

Part 3 of 3Exam Checklist & Tips

1.2.1 – Key Points for the Exam

  • Nucleus The small, dense, positively charged centre of an atom containing protons and neutrons.
  • Alpha particle A positively charged helium nucleus used as a probe in the gold foil experiment.
  • Electron A negatively charged particle found outside the nucleus.
  • Electrostatic attraction The force between oppositely charged particles that holds electrons to the nucleus.

Common Mistakes

  • Saying that most alpha particles were deflected; most passed straight through.
  • Describing the nucleus as the largest part of the atom; it is a tiny fraction of the volume.
  • Placing electrons inside the nucleus.
  • Forgetting that the deflection showed positive charge in the nucleus, not negative.

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