Radioactivity

1
Radioactivity

• Chapter 18 Nuclear Changes
• Section 1, pages 536 - 545
• Ms. Chilton

2
Large Nuclei

• Nucleus is held together by strong force
• Large nuclei tend to be unstable because the
  force is not strong enough to hold it together
• These nuclei break apart and decay
• All nuclei with 83 or more protons are
  radioactive
• Almost all elements with more than 92 protons
  dont exist naturally

3
What is Radioactivity?

• When an unstable nucleus emits one or more
  particles or energy
• When these particles are emitted, the element
  changes to another isotope or to a different
  element
• Nuclear radiation refers to radiation resulting
  from nuclear changes

4
Types of Nuclear Radiation

• Alpha particles
• Beta particles
• Gamma rays

5
Alpha Particles

• Symbol is
• Actually particles made of 2 p and 2 n0, same
  as a He nucleus
• 2 charge, most massive of all nuclear radiation,
  mass 4 amu
• Do not travel far, can be stopped by a sheet of
  paper
• Can be very dangerous inside human body illness
  and disease

6
Alpha Particle Emission
7
Beta Particles

• Symbol is Greek letter, beta (ß)
• Negatively charged particles that come from the
  nucleus hmmm
• Neutron (neutral) actually decays to form a p
  and an e-, e- is ejected from nucleus
• Travel farther than a, but can be stopped by
  3 mm of Al, or 10 mm of wood
• Can cause damage inside cells

8
Beta Decay


mk M
9
Gamma rays

• Followed in tradition, named gamma, ?
• Not made of matter, no charge
• Waves of electromagnetic energy
• Usually emitted from nucleus when alpha or beta
  decay occurs
• High energy, can be stopped by 60 cm of Al or 7
  cm of Pb
• Cause less damage than alpha because it has no
  charge

10
Gamma Radiation
11
Damage Caused
12
Penetrating Power
13
Nuclear Decay

• When unstable nuclei emit alpha or beta
  particles, what changes?
• Number of protons or neutrons
• New element or new atomic mass

14
Change During Alpha Decay

• Nucleus gives up 2 p and 2 n0
• All mass numbers add up, all atomic numbers add
  up

15
Change During Beta Decay

• Neutron is changed to a proton and an electron
  and emitted
• So neutron is lost and proton is gained

16
Classwork/Homework

• WS with example problems

17
Decay Rates

• Half-life the time in which half a radioactive
  substance decays
• After 1 half-life, half of the substance is
  unchanged
• After 2 half-lives, ¼ of the substance is
  unchanged
• Half-life table on p. 226

18
Half-Life

• Measure how quickly a substance decays
• Can be anywhere between nanoseconds to billions
  of years, depending on nuclear stability
• C-14 is used to find the age of relatively recent
  materials
• C-14 is taken in in tiny fractions while alive in
  some molecules of CO2
• C-14 goes through beta decay, so ratio is
  compared in living and nonliving things

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Half-Life Example

• The half-life of iodine-131 is 8 days.
• If you start with 36 grams of I-131, how much
  will be left after 24 days?
• 36 g ? 1 half-life ? 8 days ? 18 g
• 18 g ? 2 half-lives ? 16 days ? 9 g
• 9 g ? 3 half-lives ? 24 days ? 4.5 g

20
Classwork/Homework

• WS Nuclear Decay
• Half-Life problems on the back