Chapter 3

1
Chapter 3Atomic Structure

• Honors Special Topic
• Ch. 3-4 Changes in the Nucleus

2
OBJECTIVES

• Describe the changes that accompany nuclear
  reactions.
• Define radioactivity.
• Provide examples of nuclear equations and
  applications.

3
Chapt. 3-4 Changes in the Nucleus

• Chemical reactions involve electrons, but nuclei
  also can also change.
• Nuclear reactions change the composition of the
  atom.
• Radioactivity the spontaneous emission of
  radiation from an atom.
• Radioisotope an isotope that has an unstable
  nucleus and undergoes radioactive decay.
• Alpha, beta and gamma radiation are produced by
  nuclear changes (radioactive decay).

4
Nuclear Stability

• Recall our discussion about isotopes?
• Why dont the nuclei, which contain positively
  charged protons in a very confined space, fall
  apart spontaneously?
• Strong nuclear force (resulting from the
  neutrons) are the nuclear glue.
• Do atoms contain the same number of protons and
  neutrons?
• Stable nuclei for elements 1 20 generally have
  equal numbers of protons and neutrons. (n/p
  1/1)
• As the atom gets larger, more nuclear glue is
  needed to gain stability. (Additional neutrons
  needed 1.5/1. See Fig. 3-28 for the band of
  stability.)
• Beyond bismuth (Z 83), NO NUMBER of neutrons
  can hold the nucleus together.
• All atoms with Z gt83 are radioactive!

5
Nuclear Stability (contd)

• Over 1500 nuclei are known, but only 264 are
  stable!
• These 264 nuclei are unchanged with time.
• Lead-206 (124 neutrons 82 protons) is stable
  with a n/p ratio of 1.5.
• Transmutation the conversion of an atom of one
  element into an atom of another element.
• Occurs by spontaneous radioactive decay of a
  nucleus, or
• by artificial means (synthesis).
• Too many neutrons can make an atom unstable as
  well.
• Neutrons decay into a proton and an electron.

6
Types of Radioactive Decay

• Alpha Decay (a)
• Alpha particles are just helium nuclei.
• Mass number 4 (4 amu)
• Charge 2
• Low penetration power (Paper clothing stop
  them.)
• Beta Decay (ß)
• Beta particles are just electrons.
• Mass number 0 (1/1837 amu)
• Charge -1
• Medium penetration power (Metal foil stops them).
• Gamma Decay (?)
• Gamma radiation is high energy electromagnetic
  radiation.
• Mass number 0
• Charge 0
• High penetration power (Thick lead shield stops
  them.)

7
Nuclear Equation Rules

• The sum of the mass numbers and atomic numbers
  are the SAME before and after a nuclear reaction.
• Electrical charge of alpha particles is generally
  omitted.
• Electrical charge of beta particles is shown as a
  subscript (where Z is usually shown).

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Nuclear Equation Examples

• Examples
• Alpha decay of uranium-238
• Alpha decay of gold-185
• Beta decay of carbon-14
• Beta decay of francium-223
• Gamma rays often accompany alpha and beta
  disintegration of a nucleus.
• Thorium-230 ? Radon-226 a ?
• Thorium-234 ? Protactinium-234 ß ?
• Rutherford bombarded nittrogen-14 with a-
  particles to form F-18, leading to O-17 proton.
• Led to discovery of the proton!

9
Problems

• A nuclear reaction produced magnesium-24 and beta
  radiation. What nucleus was responsible for
  this?
• (Sodium-24)
• Mercury-200 and an alpha particle result from the
  radioactive decay of what nucleus?
• (Lead-204)
• Sheet 3-4 PP (H/W)

10
Class Activity
From Chemistry, Wilbraham, et al.,
Prentice-Hall, 2002, page 840.

• Collect 128 pennies, container, paper, graph
  paper, calculator, pen/pencil.
• Make two-column table with TRIAL and Number
  of Heads.
• Place pennies in container shake them up.
• Pour pennies onto desk. (Keep them under
  control!)
• Pick out count Heads and set them aside
  record data.
• Place remaining pennies in container and shake
  them repeat the process five times.
• Repeat the entire experiment two more times.
• Graph Heads (y-axis) vs. Trial and note
  shape.
• Add new column Log Heads and finish the
  table.
• Graph Log Heads vs. Trial and note shape.

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Half-life (t1/2)

• Half-life (t1/2) the time required for one-half
  of the atoms of a radioactive isotope
  (radioisotope) to emit radiation and decay to
  products.
• Simulating Radioactive Decay
• You just won 1,000, but
• you can only spend half of it in month 1
• half of the remainder in month 2, etc.
• After how many months would you be left with less
  than 1?
• What is the half life for this prize?

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Common Half-Lives Radiation
ISOTOPE HALF-LIFE RADIATION
Carbon-14 5,730 years Beta
Potassium-40 1,25 X 109 years Beta, gamma
Radon-222 3.8 days Alpha
Radium-226 1,600 years Alpha, gamma
Thorium-230 75,400 years Alpha, gamma
Thorium-234 24.1 days Beta, gamma
Uranium-235 7.0 X 108 years Alpha, gamma
Uranium-238 4.46 X 109 years Alpha
From Chemistry, Wilbraham, et al.,
Prentice-Hall, 2002, page 847.
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Applications of Nuclear Reactions

• Dating of ancient artifacts (Carbon-14).
• Smoke detectors (Americium-241).
• Radioactive tracers in medicine (Iodine-131,
  barium-140, phosphorus-32).
• Cancer treatment (Cobalt-60).
• Electricity generation (Uranium-235).
• Artificial (lab-made) elements (beyond Z 92).
• Bombs (Uranium-235).
• Fusion (Combining two small nuclei to form a
  large nucleus.)
• Interested in learning more? See Chapter 24.

14
OBJECTIVES

• Describe the changes that accompany nuclear
  reactions.
• Define radioactivity.
• Provide examples of nuclear equations and
  applications.