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Particle Nature of The Nucleus

Particle Nature of The Nucleus. -All nuclei have mass that is multiple of specific number. -t.f. nucleus made of smaller particles: protons, neutrons = nucleons. Mass or Nucleon Number. Nucleons live in nucleus -protons and neutrons.

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Particle Nature of The Nucleus

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  1. Particle Nature of The Nucleus • -All nuclei have mass that is multiple of specific number. • -t.f. nucleus made of smaller particles: protons, neutrons = nucleons.

  2. Mass or Nucleon Number • Nucleons live in nucleus -protons and neutrons. • Mass number (A) is equal to the total all neutrons and protons in the nucleus. • If p+ and no = atomic weight 1, the mass number gives the atomic weight in amu - u. • Notation • A - Mass number A X • Z - atomic proton# Z

  3. Isotopes • Most elements have a few different isotopes. • Isotopes of the same substance have equal proton numbers, but different numbers of neutrons. • For the same element, the Z number must always be the same but the A number may vary. • Electron configuration determines chemical properties. • Different isotopes of the same element behave the same way chemically.

  4. Nuclides • Nuclide is a distinct type of atom or isotope with specific number of p+ and no. Ex: C-14. • Nuclides are defined by many different aspects, such as half life, mode of decay, percent abundance, and so on. • Isotopes are sets of nuclides having the same number of p+, but different number of no - . Each individual isotope is a separate nuclide.

  5. Calculations of nucleons: • For a given isotope: protons (Z) + neutrons = mass number (A) • Its easy to find the number of no in isotope. A – Z is total neutrons. • 1. Find the number of no in 37 Cl 17

  6. Isotopes 2: Which of the following gives the correct number of electrons, protons and neutrons in the neutral atom 6529Cu? SOLUTION: A = 65, Z = 29, so N = A – Z = 65 – 29 = 36. Since it is neutral, the number of electrons equals the number of protons = Z = 29.

  7. Isotopes 3. Ag-102, Ag-103 and Ag-104 are all isotopes of the element silver. Which one of the following is a true statement about the nuclei of these isotopes? A. All have the same mass. B. All have the same number of nucleons. C. All have the same number of neutrons. D. All have the same number of protons. SOLUTION: Isotopes of an element have different masses and nucleon totals. Isotopes of an element have the same number of protons, and by extension, electrons. This is why their chemical properties are identical.

  8. Mass Spectrometer Isotopes The mass spectrometer, strips the atom of electrons & accelerates nucleus through a pd into a magnetic field. H nuclei had 3 different masses: The charge of the hydrogen nucleus is e, scientists postulated the a neutralparticle: the neutron, which had mass without charge. Fc = FB mv2/r = qvB mv/r = qB m = qBr/v

  9. 4: Track X shows the deflection of a singly-charged carbon-12 ion in the deflection chamber of a mass spectrometer. Which path best shows the deflection of a singly- charged C-14 ion? Assume both ions travel at the same speed. SOLUTION: Since carbon-14 is heavier, it will have a bigger radius than carbon-12. Since its mass is NOT twice the mass of carbon-12, it will NOT have twice the radius.

  10. Strong Nuclear Force • A nucleus is ~10-15 m in diameter. • The Coulomb repulsion between protons is enormous. • Most nuclei do NOT spew out their protons, there must be a nucleon force that acts within the confines of the nucleus to overcome the Coulomb force. • The strong nuclear force holds nucleons together. • The more nucleons, the more nuclear force. • Heating metals or shining EM radiation can strip e- from atom. Cannot pull apart nucleus with so little energy.

  11. Radioactive Decayoccurs when nucleus cannot hold together • Nucleus emits particles and/or energy 1896 Antoine Becquerel discovered that certain U salts emitted rays that could penetrate dark paper to expose a photographic plate. Called penetrating rays radiation.

  12. Two types of Decay • Artificial/Induced • Natural Spontaneous • Unstable Nucleus • Bombard Nucleus with a particle.

  13. Particles Emitted From Nucleus include: • Alpha • Beta • Positron • Gamma • Neutrino • Antineutrino • Photon

  14. Spontaneous Radioactive Decay is random process • The type of decay & rate of decays do not depend on pressure, temperature, chemical bonds.

  15. Alpha Particles Rays a rays are helium nuclei, (2p+ and 2no), that are emitted from nucleus. They are positively charged since e- missing. When nucleus is too large, it may emit alpha particle.

  16. Alpha Decay of Americium-241 to Neptunium-237 He nucleus Daughter Parent

  17. a particles are easily stopped by skin or thin sheet of paper.Likely to knock e- from orbits if they hit them. a loses all its KE at once when it’s stopped.Charge = +2e.Mass = 4 units.Energy is KE = ½ mv2.

  18. Alpha a Decay Alpha a have KE ~5 MeV

  19. Beta b Particles • electrons emitted from nucleus. • More penetrating than alpha because they are smaller. • Think of b as a no that emits e-, becomes a p+.

  20. Less capable of ionizing (knocking out e-)Need a few mm of Al to stop them.Charge = -1 or -e. mass = e considered massless in mass # calcs.KE = ½ mv2. v can be sig portion of c.

  21. Positron Decay like beta decay but p+ loses b+ (positive electron )becomes a neutron:

  22. Gamma g Radiation • Highly penetrating. Can pass thru human body, concrete, lead. • Similar to x-rays & light, but from the nucleus with shorter l. • Gamma ray emission is when an excited nucleus de-excites to lower (more stable) state in the nucleus. In diagrams, a gamma ray is represented by this: • In equations = g.

  23. Gamma rays are EM waves. They have:Lowest ionizing power.No charge. No mass. Energy described by E = hf. Travel with vel of light in vacuum.No maximum stopping range.

  24. Neutrinos are particles that are emitted with beta and positron decay. No charge.Almost no mass.They do not interact with matter.

  25. How could we distinguish the different types of radiation? What could we observe?

  26. Nuclear Physics 8: Properties of Becquerel Rays 9 min. • https://www.youtube.com/watch?v=INF9y154EZA • Read Homer Section 7.1 do pg 304 #1,2

  27. Nuclear Decay Equations

  28. Products of DecayWhen a parent nucleus decays, a daughter product, is produced, a new element could be produced. We can identify products by balancing mass and atomic numbers. 30

  29. 1. U – 232 decays by alpha emission. Write the nuclear equation and determine the daughter of the decay.

  30. alpha2324A U He + X922ZX will have 4 less nucleons than U-232.X will have 2 less protons than U-232.

  31. Daughter will be: 228 X 90 Look up on periodic table element with 90 p+ (Thorium). Answer is: 228 Th 90 33

  32. Positron/Beta Decay • During beta decay a beta, b- (e-) or a positron b+ (+e) is emitted from the nucleus. • When an e- (b-) emitted a no changes to a p+, the atomic number increases by 1. • When an e+ (b+) emitted a p+ changes to a no, the atomic number decreases by 1.

  33. A neutrino v or antineutrino v is also emitted in positron or beta decay. b+ decay emits neutrino v. b- decay emits an antineutrino v.

  34. Positron Decaylike beta decay but: neutrino • Proton no + b+ + n A positron particle b+ is the antimatter e-. • 90 Ru b+ + ___ + __. • 44 +1 • 90 Ru b+ + 90 Tc + n. • 44 +1 43

  35. 3. C-14 undergoes beta decay. Write the decay equation and identify the daughter.

  36. 14 0 A C e + X + v. 6 -1 ZA beta particle/e- has atomic number –1 and is considered massless.

  37. Balance – make right side add up to left side. 14 0 A C e + X + v. 6 -1 ZA will be 14. Z will be ?? 14 N 7

  38. 4. Radium-226 decays by alpha emission. Write the decay equation and identify the daughter.

  39. 226 Ra X + 4He88 2 Mass X = 226 – 4 = 222. Atomic number Z 88 – 2 = 86. 226 Ra + 4He88 2 222 Rn86

  40. 5. Sulfur 35 emits b- particles when it decays. Write the decay equation and identify the daughter.

  41. 35 S 0 e + A X + v.16-1Z Atomic number X is 17 since neutron went to proton, so daughter is: 35 17Cl.

  42. 6. Complete the equation: • 23 Mg b+ + __ __ + __. • 12 +1 __ • 23 Mg b+ + 23Na+ n. • 12 +1 11

  43. Gamma g radiation • Often emitted in another type of decay.

  44. Nuclear Stability The more stable the nucleus, the less likely that it will decay.

  45. Electrostatic repulsion in the positively charged nucleus makes it want to decay.The strong nuclear force holds the nucleus together.

  46. The extra neutrons increase the strong force & help shield against electric repulsion.

  47. As more p+ added to nucleus more no needed for stability (for nucleus not to decay). There seems to be a ratio between p+ and no for each element to maintain enough strong force to keep nucleus from flying apart.In general heavier elements require more no.p+– no ratio determines stability.

  48. Nuclides above the band are too large -decay by a.To the left b- decay occurs. Nuclides below the band have too few no, positron decay occurs. A p+ becomes a no.

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