Atoms, nuclei and nuclear change

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レッスンノート

Big idea: atoms, nuclei and nuclear change

  • Key concept: Systems. A tiny, positively charged nucleus accounts for large deflections of a few incident α particles. Most particles pass through because an atom is mostly empty space.
  • Related concepts: Models and evidence. Use a scientific explanation to make predictions, then test it against observations.
  • Global context: Scientific and technical innovation. Scattering evidence lets scientists test models of structures too small to observe directly.

Atomic Structure

  • Atoms consist of a small, dense, positively charged nucleus surrounded by negatively charged electrons in orbit.
  • Atom radius ≈ 1 × 10⁻¹⁰ m; nucleus radius is over 10,000 times smaller but contains almost all the mass.
  • A neutral atom has equal numbers of protons and electrons.
  • Ions are formed when an atom loses or gains electrons: losing electrons → positive ion; gaining electrons → negative ion.

Diagram of an atom with a positively charged nucleus and negatively charged electrons orbiting it.

Diagram of an atom with a positively charged nucleus and negatively charged electrons orbiting it.

Rutherford's Alpha Scattering Experiment

  • In 1909, Rutherford, Geiger and Marsden fired α-particles at thin gold foil.
  • Most α-particles passed straight through → atom is mostly empty space.
  • Some were deflected → repelled by a small, positively charged nucleus.
  • A very few bounced back → nucleus is tiny and contains most of the mass.
  • This disproved the Plum Pudding model and provided evidence for the nuclear model.

Rutherford's Alpha Scattering Experiment

Rutherford's Alpha Scattering Experiment

The Nucleus

  • The nucleus contains protons (positive charge) and neutrons (no charge); overall positive charge.
  • Proton number (Z) = number of protons = atomic number; determines the element.
  • Nucleon number (A) = total number of protons and neutrons (mass number).
  • Number of neutrons = nucleon number − proton number.

Structure of the atom

Structure of the atom

Nuclide Notation

  • Nuclide notation: \ce{AZX} where X = element symbol, A = nucleon number, Z = proton number.
  • Example: \ce{19779Au} has 79 protons, 118 neutrons (197−79), and 79 electrons.
  • A nuclide is a group of atoms with the same number of protons and neutrons.

Isotopes

  • Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons.
  • They have the same Z but different A.
  • Example: \ce{23592U} and \ce{23892U} are isotopes of uranium.
  • Some isotopes are stable and others are radioactive. Nuclear stability depends on the balance of protons and neutrons and other nuclear properties; being an isotope does not itself mean being unstable.

Protons, Neutrons & Electrons

  • Relative charges: proton = +1, neutron = 0, electron = −1.
  • Relative masses: proton = 1, neutron = 1, electron = negligible (≈1/2000).
  • Nuclear charge = number of protons × (+1).
  • Relative mass of atom ≈ nucleon number (mass of electrons negligible).

Nuclear Fission

  • Nuclear fission is the splitting of a heavy nucleus into smaller nuclei, often with neutrons and energy released; neutron absorption can trigger it.
  • A neutron collides with the nucleus, which splits into daughter nuclei and 2–3 neutrons, releasing energy and gamma rays.
  • Example: \ce{23592U + 10n → 9236Kr + 14156Ba + 310n + energy}.
  • The total nucleon and proton numbers are conserved; mass decreases slightly as mass is converted to energy.
  • Used in nuclear power stations with fuels like uranium-235 and plutonium.

Nuclear Fusion

  • Nuclear fusion is the joining of two light nuclei to form a heavier nucleus.
  • Occurs in stars (e.g., hydrogen fusing to helium) and requires extremely high temperature and pressure.
  • Example: \ce{21H + 11H → 32He + energy}.
  • Mass of product is slightly less than reactants; mass converted to energy releases huge amounts (1 kg hydrogen ≈ 10 million kg coal).

Think like a scientist

  • Analyse a teacher-provided scattering simulation to compare particle deflections at different approach distances from a nucleus.
  • Comparison: the approach distance from the nucleus. Outcome: the scattering angle.
  • Control: keep particle energy and nuclear charge constant. Explain why this makes the comparison fairer.
  • Evidence: Use a consistent method, repeated observations where appropriate and a table with labelled quantities and units. Keep unexpected results and investigate their cause.
  • Safety: Practical activities need teacher supervision and an appropriate risk assessment. Use the provided data or simulation where the investigation specifies it.
  • Inquiry task: State a testable question, predict the outcome using the science, then explain how your observations would support or challenge the prediction.

Evaluate the science

  • Scattering evidence lets scientists test models of structures too small to observe directly.
  • A model is judged by predictions and evidence; an attractive drawing alone does not establish its accuracy.
  • Evaluation task: Link your conclusion to evidence, identify a limitation and suggest a specific improvement. Distinguish a measured result from an explanation of its cause.

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練習問題

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  1. 1.Which statement about the three isotopes of hydrogen is correct?

    Easy
    • AThey must have different numbers of electrons orbiting their nucleus.
    • BThey must all have one proton in their nuclei.
    • CThey must all have one neutron in their nuclei.
    • DThey must all have the same number of nucleons in their nucleus.

  2. 2.An atom consists of electrons orbiting a nucleus. The nucleus contains protons and neutrons. For the atom to be neutral, which statement must be correct?

    Easy
    • AThe number of electrons is equal to the number of protons.
    • BThe number of protons is equal to the number of neutrons.
    • CThe number of electrons, protons and neutrons are all different.
    • DThe number of electrons is equal to the number of neutrons.

  3. 3.The charge on an electron is -e. Which answer correctly describes the charge of a proton and the charge on a neutron?

    Easy
    • AProton: 0, Neutron: +e
    • BProton: +e, Neutron: -e
    • CProton: +e, Neutron: 0
    • DProton: -e, Neutron: +e

  4. 4.An isotope of uranium is represented by the symbol 92235U. Which of the answers below could be a different isotope of uranium?

    Medium
    • A91235U
    • B92238U
    • C23592U
    • D90235U

  5. 5.Atom X loses an electron to form an ion. Atom Y gains an electron to form an ion. Which row in the table is correct?

    Easy
    • Acharge on ion of X: positive, charge on ion of Y: positive
    • Bcharge on ion of X: positive, charge on ion of Y: negative
    • Ccharge on ion of X: negative, charge on ion of Y: positive
    • Dcharge on ion of X: negative, charge on ion of Y: negative

  6. 6.Where is the positive charge in an atom?

    Easy
    • Aon the orbiting particles
    • Bin the nucleus
    • Cin the space between the nucleus and the orbiting particles
    • Dspread throughout the atom

  7. 7.Which row correctly identifies the relative charges of the three types of particle in an atom?

    Easy
    • Aproton: +1, neutron: -1, electron: 0
    • Bproton: +1, neutron: 0, electron: -1
    • Cproton: -1, neutron: +1, electron: 0
    • Dproton: -1, neutron: 0, electron: +1

  8. 8.How many neutrons are in a nucleus of aluminium? The chemical symbol is 1327Al.

    Easy
    • A13
    • B27
    • C14
    • D40

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