Chapter Objectives and Summaries
for the Oxford University Press text by Walding, Rapkins and Rossiter:-
" NEW CENTURY Senior Physics - Concepts in Context "
CHAPTER 27 Atomic structure
Knowledge of subject matter
- Give an outline of the development of our knowledge of the atom.
- Distinguish between the atomic models of Thomson, Rutherford and Bohr.
- State the purpose and results of Thomson's and Millikan's experiments.
- Explain what radioactivity is and say how it was discovered.
- Describe the structure of the atom. List the properties of protons, neutrons and electrons.
- Define the terms 'atomic number' and 'mass number'.
- Explain what an isotope is. Give examples.
- Describe the various forces that exist in an atom. Explain what is meant by nuclear binding energy.
- Classify the basic particles of an atom based on their properties.
- Investigate the historical development of the model of the atom
COMPLEX REASONING PROCESSES
- Solve novel problems involving atomic structure, composition and binding energies.
- Critically analyse the relevance of the different models of the atom
CHAPTER 27 SUMMARY
- Early theories on the structure of matter were those of Democritus (atoms) and Aristotle (the four elements).
- Modern atomic theories were developed by Dalton, Thompson, Rutherford and Bohr.
- Dalton's theory stated that (i) elements were made up of tiny particles called atoms, (ii) each element had its own type of atom, (iii) atoms cannot be created nor destroyed, (iv) atoms are indivisible.
- Thompson discovered electrons and measured the e/m ratio using Crookes' discharge tubes and noting the deflection in both electric and magnetic fields.
- Millikan used charged oil drops in an electric field to determine the charge on an electron and hence its mass.
- Radioactivity is the process whereby atoms emit particles of high energy from their nuclei.
- The main experimentalists in the field of radioactivity were Roentgen, Becquerel and Marie and Pierre Curie.
- Nuclear radiation (alpha a, beta b, gamma g) can be classified on the basis of ionising and penetrating ability.
- Alpha and beta radiation are affected by a magnetic field; gamma is not. X-rays are similar to gamma rays in that they are electromagnetic radiation but X-rays come from the movement of electrons whereas gamma rays come from reactions within the nucleus.
- Rutherford's gold leaf experiment showed the atom to be mostly empty space with a dense positive nucleus and orbiting electrons.
- Rutherford detected the proton; Chadwick, the neutron. These are called nucleons.
- Bohr said that electrons in atoms can have only certain fixed amounts of energy. The electrons revolve around the nucleus in certain allowed orbits called energy levels. Electron can jump from one energy level to another by the absorption or emission of a bundle (quantum) of energy in the form of a photon.
- Atoms are characterised by their number of protons (their atomic number); different numbers of neutrons produces different isotopes and hence nuclides of different atomic mass. Atomic mass equals the sum of protons and neutrons.
- The mass of a nucleus is always less than the combined individual masses of its constituent nucleons. The difference in mass is called the mass defect. This represents the binding energy which is equivalent to the energy released during the formation of a nucleus and is the energy that must be applied to the nucleus to break it apart. It can be calculated using Einstein's equation E = mc².
- The stability of a nuclide depends on its binding energy per nucleon; the greater this value the more stable the nuclide.
- The SI unit of atomic mass is the unified atomic mass unit (u).
- A mass of one unit (u) is equivalent to 931 MeV of energy or 1.6606 x 10-27 kg.
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