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5.1.1Nuclear Physics

The Atom — Cambridge IGCSE Physics 0625 Revision Notes & Topical Questions

5 min read 9 parts Lesson 3 of 17 1 quiz

📚 Cambridge IGCSE Physics 0625 Hub / Topical Revision
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Lesson summary

5.1.1 The Atom — IGCSE PHYSICS notes: Describe the structure of an atom, how ions form and the alpha-particle scattering experiment.Nucleus — a small, dense, positively charged centre containing protons and neutrons.Electrons — negatively charged particles orbiting the nucleus in…

  • Empty space
  • Atomic number
  • Positive ion
  • Negative ion
  • Net charge
  • Rutherford

Part 1 of 6Structure of the atom

Structure of the atom
Nucleus
  • Nucleus a small, dense, positively charged centre containing protons and neutrons.
Electrons
  • Electrons negatively charged particles orbiting the nucleus in shells.
    • (2, then 8, then 8)
Size
  • Size an atom has a radius of about 1 × 10−10 m, and the nucleus is over 10 000 times smaller (about 10−15 m).
Mass
  • Mass almost all the mass of the atom is in the nucleus.
Empty space
  • Empty space most of an atom is empty space.
Charge
  • Charge a neutral atom has equal numbers of protons and electrons, so the charges cancel.

Part 2 of 6Particles in the atom

ParticleRelative chargeRelative massLocation
Proton+11Nucleus
Neutron01Nucleus
Electron−1Negligible (about 1/1840)Shells around the nucleus
Particles in the atom
Atomic number
  • Atomic number the number of protons identifies the element.
Neutrons
  • Neutrons equal the mass number minus the atomic number.

Part 3 of 6Atoms and ions

Ion
Ion an electrically charged atom or group of atoms formed by the loss or gain 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 of the atom or ion.

Part 4 of 6Development of the atomic model

1Dalton
  • Dalton a solid sphere.
2Thomson
  • Thomson the plum pudding model, with positive charge spread through the atom.
3Rutherford
  • Rutherford a small, dense nucleus.
4Bohr
  • Bohr electrons in shells.

Part 5 of 6Observations and conclusions (Extended)

ObservationConclusion
Most alpha particles passed straight throughThe atom is mostly empty space
Some alpha particles were deflected through small anglesThe nucleus is positively charged and repels the positive alpha particles
A few alpha particles bounced backThe nucleus is tiny and contains most of the mass
  • Result
    • the bouncing back could not be explained by the plum pudding model, so it was the first evidence of the nuclear atom.

Part 6 of 6More from the specification

More from the specification
Atomic composition and nucleon counting
  • Atomic composition and nucleon counting subtracting atomic number from mass number provides a straightforward means to determine the neutron count in atoms.
Nuclide notation
  • A nuclide is written as A over Z with the symbol, where A is nucleon number and Z is proton number, e.g. hydrogen-1.
Isotopes
  • Atoms of the same element with the same protons but different neutrons, such as hydrogen-2 (deuterium) and hydrogen-3 (tritium).
Nuclear fission
  • A large unstable nucleus splits into smaller nuclei when hit by a neutron, releasing energy and more neutrons.
Nuclear fusion
  • Light nuclei such as hydrogen join to form a heavier nucleus, releasing energy; this powers stars.
Nuclear equations
  • Reactants and products must balance in both nucleon number and proton number.
Nuclide notation
  • A nuclide is written as A over Z with the symbol, where A is nucleon number and Z is proton number, e.g. hydrogen-1.
Isotopes
  • Atoms of the same element with the same protons but different neutrons, such as hydrogen-2 (deuterium) and hydrogen-3 (tritium).
Nuclear fission
  • A large unstable nucleus splits into smaller nuclei when hit by a neutron, releasing energy and more neutrons.
Nuclear fusion
  • Light nuclei such as hydrogen join to form a heavier nucleus, releasing energy; this powers stars.
Nuclear equations
  • Reactants and products must balance in both nucleon number and proton number.
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.
Nuclide
  • Nucleus with a specific number of protons and neutrons.
Isotopes
  • Nuclei with the same proton number but different neutron numbers.

Key terms

TermMeaning
NucleusSmall, dense, positively charged centre of the atom.
ElectronNegative particle orbiting the nucleus.
IonAtom or group of atoms that has gained or lost electrons.
Alpha scatteringExperiment firing alpha particles at gold foil that showed the nuclear atom.
Plum pudding modelEarlier model with positive charge spread through the atom.

Avoid these mistakes

1
Saying electrons are in the nucleus — the nucleus contains protons and neutrons only.
2
Confusing the atom with the nucleus — the atom is the nucleus plus electrons.
3
Saying most of an atom's mass is in the electrons — it is in the nucleus.
4
Saying alpha particles bounced back because the atom is full of positive charge — it is because the nucleus is tiny and dense.

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