Development of models of the atom has resulted from both experimental investigations and hypotheses based on theoretical considerations.
- A key piece of experimental evidence supporting the nuclear model of the atom was a discovery by Chadwick in 1932.
- An aspect of the experimental design is shown.
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- What was the role of paraffin in Chadwick's experiment? (2 marks)
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- How did Chadwick's experiment change the model of the atom? (3 marks)
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- What was the role of paraffin in Chadwick's experiment? (2 marks)
- Explain how de Broglie's hypothesis regarding the nature of electrons addressed limitations in the Bohr-Rutherford model of the atom. (4 marks)
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a.i. Role of paraffin wax:
→ Paraffin wax is a rich source of protons.
→ When the paraffin was placed in front of the unknown radiation, the transfer of momentum from the radiation caused protons to be emitted from the paraffin wax.
→ The emitted protons could then be detected and analysed.
→ From studying the protons ejected from the paraffin wax, Chadwick proposed the existence of the neutron.
a.ii. Changes to the model of the atom:
→ Previous to Chadwick’s experiment, the model of the atom proposed by Rutherford consisted of a dense positive charge in the nucleus which was orbited by electrons.
→ In this model however, the protons did not account for the total mass of the nucleus.
→ Through using the conservation of momentum and energy in his experiment, Chadwick proposed the existence of the neutron particle which was slightly heavier than the proton.
→ The model of the atom was updated to include both protons and neutrons in the nucleus which then fully accounted for the mass of the nucleus.
b. Limitations in the Bohr-Rutherford model:
→ Rutherford’s model of the atom stated that electrons orbited the nucleus and were electrostatically attracted to the positive nucleus. This meant that the electrons were in circular motion and were constantly under centripetal acceleration.
→ However, Maxwell predicted that an accelerating charge would emit electro-magnetic radiation and in Rutherford’s model, all atoms should have been unstable as the electrons would emit EMR, lose energy and spiral into the nucleus.
→ Bohr proposed that electrons orbited the nucleus in stationary states at fixed energies with no intermediate states possible where they would not emit EMR but provided no theoretical explanation for this.
De Broglie’s hypothesis:
→ De Broglie proposed that electrons could exhibit a wave nature and could act as matter-waves. The electrons would form standing waves around the nucleus and would no longer be an accelerating particle which addressed the limitation of all atoms being unstable.
→ Further, De Broglie proposed that the standing waves would occur at integer wavelengths where the circumference of the electron orbit would be equal to an integer electron wavelength,
a.i. Role of paraffin wax:
→ Paraffin wax is a rich source of protons.
→ When the paraffin was placed in front of the unknown radiation, the transfer of momentum from the radiation caused protons to be emitted from the paraffin wax.
→ The emitted protons could then be detected and analysed.
→ From studying the protons ejected from the paraffin wax, Chadwick proposed the existence of the neutron.
a.ii. Changes to the model of the atom:
→ Previous to Chadwick’s experiment, the model of the atom proposed by Rutherford consisted of a dense positive charge in the nucleus which was orbited by electrons.
→ In this model however, the protons did not account for the total mass of the nucleus.
→ Through using the conservation of momentum and energy in his experiment, Chadwick proposed the existence of the neutron particle which was slightly heavier than the proton.
→ The model of the atom was updated to include both protons and neutrons in the nucleus which then fully accounted for the mass of the nucleus.
b. Limitations in the Bohr-Rutherford model:
→ Rutherford’s model of the atom stated that electrons orbited the nucleus and were electrostatically attracted to the positive nucleus. This meant that the electrons were in circular motion and were constantly under centripetal acceleration.
→ However, Maxwell predicted that an accelerating charge would emit electro-magnetic radiation and in Rutherford’s model, all atoms should have been unstable as the electrons would emit EMR, lose energy and spiral into the nucleus.
→ Bohr proposed that electrons orbited the nucleus in stationary states at fixed energies with no intermediate states possible where they would not emit EMR but provided no theoretical explanation for this.
De Broglie’s hypothesis:
→ De Broglie proposed that electrons could exhibit a wave nature and could act as matter-waves. The electrons would form standing waves around the nucleus and would no longer be an accelerating particle which addressed the limitation of all atoms being unstable.
→ Further, De Broglie proposed that the standing waves would occur at integer wavelengths where the circumference of the electron orbit would be equal to an integer electron wavelength,