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The atom

The atom. orbiting electrons. Nucleus (protons and neutrons). Nuclide notation. Nucleon number (A) = number of protons and neutrons. Neutron number (N) = A - Z. 7. Li. 3. Proton number (Z) = number of protons . 7. 6. Li. Li. 3. 3. Isotopes.

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The atom

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  1. The atom orbiting electrons Nucleus (protons and neutrons)

  2. Nuclide notation Nucleon number (A) = number of protons and neutrons Neutron number (N) = A - Z 7 Li 3 Proton number (Z) = number of protons

  3. 7 6 Li Li 3 3 Isotopes It is possible for the nuclei of the same element to have different numbers of neutrons in the nucleus (but it must have the same number of protons)

  4. 7 6 Li Li 3 3 Isotopes For example, Lithium atoms occur in two forms, Lithium-6 and Lithium-7 3 neutrons 4 neutrons

  5. 1 2 3 H H H 1 1 1 Isotopes of Hydrogen The three isotopes of Hydrogen even have their own names! Hola! Mi nombre es tritium y yo soy de Madrid! They call me deuterium Hi! I’m hydrogen

  6. How do we know the structure of the atom?

  7. The famous Geiger-Marsden Alpha scattering experiment In 1909, Geiger and Marsden were studying how alpha particles are scattered by a thin gold foil. Thin gold foil Alpha source

  8. Geiger-Marsden As expected, most alpha particles were detected at very small scattering angles Thin gold foil Small-angle scattering Alpha particles

  9. Geiger-Marsden To their great surprise, they found that some alpha particles (1 in 20 000) had very large scattering angles Thin gold foil Small-angle scattering Alpha particles Large-angle scattering

  10. Explaining Geiger and Marsdens’ results The results suggested that the positive (repulsive) charge must be concentrated at the centre of the atom. Most alpha particles do not pass close to this so pass undisturbed, only alpha particles passing very close to this small nucleus get repelled backwards (the nucleus must also be very massive for this to happen). nucleus

  11. Rutherford did the calculations! Atomic nucleus had a diameter of about 10-15 m That’s 100,000 times smaller than the size of an atom(about 10-10 meters).

  12. Stadium as atom If the nucleus of an atom was a ping-pong ball, the atom would be the size of a football stadium (and mostly full of nothing)! Nucleus (ping-pong ball

  13. Forces in the Nucleus

  14. Coulomb Force in Nucleus • Repulsive force between protons + +

  15. The Strong Force The nucleons (protons and neutrons) in the nucleus are bound together by the strong nuclear force

  16. Strong Force • Acts over short distances (10-15 m) • Acts only between adjacent particles in the nucleus • Carried by “gluons”

  17. Why is a nucleus unstable? Because of the relative numbers of p and n Ex. Uranium 235 Hi! I’m uranium-235 and I’m unstable. I really need to lose some particles from my nucleus to become more stable.

  18. The unstable nucleus emits a particle to become stable Thus, the atom (nucleus) has decayed. Weeeeeeeeeeeeee!

  19. Decay of an Unstable Nucleus 1. Random – It’s going to happen, but when?! 2. Spontaneous – Not affected by temperature, pressure etc. Weeeeeeeeeeeeee!

  20. Becquerels (Bq) • The amount of radioactivity given out by a substance is measured in Becquerels. • One Becquerel is one particle emitted per second.

  21. How to detect particles? • Photographic film • Fluorescence • Geiger-Müller tube (GM tube) connected to a counter (counts the # of particles) • Use filters to distinguish alpha, beta, and gamma particles

  22. Three main types of particles can be ejected from an unstable nucleus.

  23. Alpha particles Hi! α

  24. 2+ 4 4 He α 2 2 Alpha particles • 2 protons and 2 neutrons joined together • Helium nucleus • Stopped by paper or a few cm of air • Highly ionising • Deflected by electric and strong magnetic fields

  25. 235 231 U Th 92 90 2+ 4 He 2 Alpha Decay Atomic mass goes down by 4 + Atomic number goes down by 2

  26. 235 231 U Th 92 90 4 α 2 Alpha Decay Atomic mass goes down by 4 + Atomic number goes down by 2

  27. Beta particles Yo! β

  28. Beta particles • Fast moving electrons or positrons • Stopped by about 3 mm of aluminium • Weakly ionising • Deflected by electric and magnetic fields β e β e 0 0 0 0 -1 +1 -1 +1 Beta Negative  electron Beta Positive  Positron

  29. 46 40 40 46 Ar + Sc + K Ca 20 19 21 18 Beta decay • In the nucleus a neutron[or proton] changes into an electron[positron] (the beta particle which is ejected) and a proton[neutron] (which stays in the nucleus) and an antineutrino[neutrino] • During beta decay the mass number stays the same but the proton number goes up[or down] by 1. antineutrino e +עe 0 0 0 -1 e +עe neutrino 0 0 +1 0

  30. 46 40 40 46 Ar + Sc + K Ca 20 19 21 18 Beta decay • In the nucleus a neutron[or proton] changes into an electron[positron] (the beta particle which is ejected) and a proton[neutron] (which stays in the nucleus) and an antineutrino[neutrino] • During beta decay the mass number stays the same but the proton number goes up[or down] by 1. antineutrino β+עe 0 0 0 -1 β+עe neutrino 0 0 +1 0

  31. Gamma rays Hola!

  32. Gamma rays • High frequency electromagnetic radiation • Stopped by several cm of lead • Very weakly ionising • NOT affected by electric or magnetic fields

  33. 235 231 231 U Th* Th 92 90 90 Gamma rays Usually associated with alpha decay Excited nucleus relaxes by releasing a gamma ray. α + +

  34. Biological Effects of Localized Radiation Exposure • Short Term • Low dosage – skin reddening • High dosage – skin or tissue necrosis • Long Term – through direct or indirect actions • Damages or mutates DNA and other cells (specific concern: somatic cells) • Potential cancer causing

  35. Best Practice: Prevention! • Limit Radiation Concentration (in labs) • Limit Exposure Time • Increase Distance • Use Appropriate Shielding Materials

  36. ½ - life • The decay of a single nuclei is totally random • However, with large numbers of atoms a pattern does occur

  37. Number of nuclei undecayed time half-life (t½) ½ - life • This is the time it takes half the nuclei to decay

  38. ½ - life • This is the time it takes half the nuclei to decay Number of nuclei undecayed time half-life (t½)

  39. ½ - life • This is the time it takes half the nuclei to decay Number of nuclei undecayed A graph of the count rate against time will be the same shape time half-life (t½)

  40. Different ½ - lives • Different isotopes have different half-lives • The ½-life could be a few milliseconds or 5000 million years! Number of nuclei undecayed time half-life (t½)

  41. Decay Constant (λ) • The probability that an atom will decay • Remember that the decay of a single nuclei is totally random • Activity = the decay constant times the number of atoms • Equation  A = λ N

  42. Determining ½ - lives • For short half-lives • Use a GM-tube to measure initial activity, and then measure the time it takes for that activity to decrease by a half. • That time is the half-life! • For long half-lives • Use a GM-tube to measure the initial Activity • Measure the mass of the sample • Calculate the # of atoms using molar mass and Avogadro’s number • Calculate λ [decay constant] from A0 = λ N0 • Calculate half-life  t1/2 = ln(2) / λ

  43. Nuclear Reactions

  44. Unified mass unit (u) • Defined as 1/12 of the mass of an atom of Carbon-12 u = 1.6605402 x 10-27 kg

  45. Energy mass equivalence • E = mc2 • E = 1.6605402 x 10-27 x (2.9979 x 108)2 • E = 1.4923946316 x 10-10 J • Remembering 1 eV = 1.602177 x 10-19 J • 1 u = 931.5 MeV • The electron-Volt is a convenient unit of energy for nuclear physics

  46. Mass defect For helium, the mass of the nucleus = 4.00156 u But, the mass of two protons and two nuetrons = 4.0320 u!!!! Where is the missing mass?

  47. Mass defect The missing mass (mass defect) has been stored as energy in the nucleus. It is called the binding energy of the nucleus. It can be found from E = mc2

  48. Mass defect calculation • Find the mass defect of the nucleus of gold, 196.97 - Au

  49. Mass defect calculation • The mass of this isotope is 196.97u • Since it has 79 electrons its nuclear mass is 196.97u – 79x0.000549u = 196.924u

  50. Mass defect calculation • The mass of this isotope is 196.97u • Since it has 79 electrons its nuclear mass is 196.97u – 79x0.000549u = 196.924u • This nucleus has 79 protons and 118 neutrons, individually these have a mass of 79x1.0007276u + 118x1.008665u = 198.080u

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