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Natural Science - Year II

Unit 53: arie and Pierre Curie and Radioactivity

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Science Weblecture for Unit 53

This Unit's Homework Page History Lecture Science Lecture Lab Parents' Notes

Science Lecture for Unit 53: Radioactivity

For Class



The work of the Curies and Thomson led to a new understanding of atomic components. In the historical section of our next unit, we'll see how Rutherford and Bohr determined the basic structure of the atom, but we are going to start looking at the parts of the atom now.

The Atom

Look at this brief introduction to the particles in the atom and notation describing atoms. [1 short web page, some graphics]

  • Compare the masses of the proton, electron, and neutron.
  • Compare the charges on the proton, electron, and neutron.
  • What is the atomic number of an element?
  • What is the atomic mass number of the element?
  • What is an isotope?

The number of protons in an atom determines what element the atom is. Hydrogen has one proton, helium two, lithium three, and so on. Each proton carries a positive single charge, so a neutral atom has the same number of negativelycharged, light-weight electrons as protons to balance this charge out.

Elements other than hydrogen also have neutrons in the nucleus. These particles with the nearly same mass as a proton, but no electrical charge, appear to be a kind of combination of proton and neutron. The more protons there are in an element, the more neutrons there will be. The number of neutrons determines the isotope of the element. For example, all carbon atoms have 6 protons (that makes them carbon), and in most carbon atoms, there are six neutrons. Counting both protons and neutrons, there are 12 nucleons in these atoms, so these are carbon-12 atoms. However, some carbon atoms have 8 neutrons, for a total of 6 + 8 = 14 nucleons. These are a different isotope, carbon-14.

The number of neutrons determins the stability of the atom. If the neutrons can't balance out the play of forces inside the atom, the atom breaks apart with a burst of energy in radioactive decay. Some combinations of neutrons and protons are more stable than others: carbon-12 is a stable isotope, but carbon-14 is not, and tends to undergo decay in time.

Types of Decay

There are three majory types of decay events.

Rates of Decay

Different unstable isotopes decay at different rates. Because the stability of the atom depends from moment to moment on the particular combination and position of its protons and neutrons, this stability is constantly changing. We can't predict exactly when a given atom will decay, but after collecting statitistical data from many thousands of experiments and observations, we can predict the likelihood that some atoms in a sample will decay in a given time period. The most common way to describe these radiation rates is by using half-lives.

Suppose that you have 100 atoms of carbon-14. From our experimental data, we can predict that about half the atoms, 50 of them, will decay at some point in the next 5730 years. We don't know which ones will decay, and we could get to the end of this period with several more or less, but the chance is close to 100% that between 45 and 55 of our 100 atoms will have "convulsed" and gone from carbon-14 to nitrogen-14. During the next 5730 years, half (25) of what was carbon-14 at the beginning of this period (50) will also decay, so at the end of two half-lives, the remaining carbon-14 is at 25% of what we started with. After another half-life period, we will have 1/2 of 25 = 13 (no half-atoms allowed!) carbon-14 atoms left.

Unfortunately, the simulation I used to use here required java applet support — which no modern browser still offers. There is a reasonably good video α, β, and γ Radiation ] on YouTube.

Then use the Half Life Virtual Lab to determine the number of atoms left after each 1000 period from an initial 100 atoms for four elements with different half-lives.

  • Which element has the longest half life? How long is it?
  • Which element has the shortest half life? How long is it?

Study/Discussion Questions

Further Study On your Own (Optional)