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5.1Nuclear Physics

The Nuclear Model of the Atom — Cambridge IGCSE Physics 0625 Revision Notes & Topical Questions

10 min read 25 parts Lesson 2 of 17 Unit quiz

📚 Cambridge IGCSE Physics 0625 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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Lesson summary

5.1 The Nuclear Model of the Atom (Part 1) — IGCSE PHYSICS notes: Describe the nuclear model of the atom, the alpha scattering evidence for it, and use proton, nucleon and neutron numbers.Nuclear model — an atom has a small, dense, positively charged nucleus with negatively charg…

  • Most pass straight through
  • Some are deflected
  • A few bounce back
  • Neutral atom
  • Positive ion
  • Negative ion

Part 1 of 6Structure and size of the atom

Structure and size of the atom
Nuclear model
  • Nuclear model an atom has a small, dense, positively charged nucleus with negatively charged electrons orbiting it.
Size
  • Size an atom has a radius of about 1 × 10−10 m, so about a hundred million atoms fit side by side across a thumbnail.
Mass
  • Mass the nucleus has a radius over 10 000 times smaller than the atom, but it contains almost all of its mass.

Part 2 of 6Atoms and ions

Neutral atom
  • Neutral atom the number of protons equals the number of electrons.
Positive ion
  • Positive ion an atom that loses electrons has more protons than electrons.
Negative ion
  • Negative ion an atom that gains electrons has more electrons than protons.
Net charge
  • Net charge the overall charge; an atom with 5 protons, 5 neutrons and 6 electrons has a net negative charge.

Part 3 of 6Composition of the nucleus

Composition of the nucleus
Particles
  • Particles protons are positive and neutrons are neutral, so the nucleus is positively charged overall.
Terminology
  • Terminology the atom is the nucleus plus its electrons; the nucleus is just the protons and neutrons.
Proton number Z
  • Proton number Z the number of protons (atomic number);
    • it decides which element the atom is and equals the number of electrons in a neutral atom.
  • Hydrogen has Z = 1, sodium 11 and uranium 92.
Nucleon number A
  • Nucleon number A the total number of protons and neutrons (mass number); electron mass is negligible.
Neutrons
  • Neutrons number of neutrons = A − Z.

Part 4 of 6Nuclide notation

Form
  • AZX, with the nucleon number on top and the proton number below the element symbol X.
Nuclide
  • Nuclide a type of atom with a particular number of protons and neutrons.

Part 5 of 6Comparing the plum pudding and nuclear models

FeaturePlum pudding modelNuclear model
Positive chargeSpread through the whole atomConcentrated in a tiny nucleus
ElectronsEmbedded in the positive materialOrbit the nucleus in mostly empty space
Alpha scattering resultPredicts only small deflectionsExplains the large deflections and rebounds

Part 6 of 6More from the specification

More from the specification
Subatomic structure deduction
  • Rutherford alpha scattering experiments provided a revolutionary means to discover the small, dense, positively charged atomic nucleus.
Rutherford alpha scattering experiment
  • Alpha particles fired at thin gold foil mostly passed straight through, with a few deflected at large angles, proving atoms consist of a tiny, dense positive nucleus surrounded by empty space.
Nuclide notation and composition
  • A nuclide is denoted by atomic number (Z, protons) and mass number : mass number = protons + neutrons.
    • (A, total nucleons)
Nuclear fission vs nuclear fusion
  • Fission splits heavy unstable nuclei (uranium-235) into smaller fragments releasing energy;
    • fusion combines light nuclei (hydrogen) under extreme temperature and pressure.
Nuclear fission and fusion energy
  • Nuclear fission and fusion energy nuclear fission splits heavy parent nuclei into lighter products,
    • while fusion joins light nuclei, releasing massive energy.
Isotopes and nuclide notation
  • Isotopes and nuclide notation isotopes possess identical atomic numbers but different mass numbers, written in standard AZX nuclide notation.
Subatomic particle relative properties
  • Subatomic particle relative properties atomic structure consists of positively charged protons, neutral neutrons in the nucleus,
  • and orbiting negative electrons of negligible mass.
Relative charge
  • Charge compared with the fundamental charge 1.6 × 10−19 C.
Nuclide notation
  • AZX.
Nuclide equation
  • An equation showing nuclei and particles in a nuclear reaction.
Fission
  • Splitting of a large nucleus releasing energy.
Fusion
  • Joining of light nuclei into a heavier one.
Isotopes
  • Nuclei with the same proton number but different neutron numbers.

Key terms

TermMeaning
NucleusThe small dense central part of an atom made of protons and neutrons.
Proton number (Z)The number of protons in the nucleus.
Nucleon number (A)The number of protons plus neutrons in the nucleus.
IonAn atom or group of atoms with a net charge from losing or gaining electrons.
Nuclide notationWriting an atom as AZX.
Alpha scatteringThe experiment in which alpha particles are fired at gold foil, giving evidence for the nucleus.

Avoid these mistakes

1
Saying electrons are in the nucleus — they orbit outside it.
2
Using the electron number for the nucleon number — nucleon number counts protons and neutrons only.
3
Mixing up A and Z in the notation — A is on top.
4
Saying the atom is mostly solid — it is mostly empty space.

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