1. Chapter 7 - Radioactivity
Science 10

2. 7.1 Atomic Theory and Radioactive Decay
Radioactivity is the release of high energy particles or waves being emitted (released) from the decay (breakdown) of an unstable nucleus of an atom
Natural background radiation exists all around us.

3. Radiation High-energy rays/particles emitted by radioactive sources.
Radioactive materials are in many things:
X-rays, radiation therapy and electricity generation are beneficial.
High energy particles and waves can do damage to DNA in our cells.
When atoms lose high energy particles and waves, ions or even new atoms can be formed.

4. The Electromagnetic Spectrum
All of the radiation found around in our environment is called natural background radiation

5. History: Roentgen
Roentgen named X-rays with an “X” 100 years ago because they were previously unknown.
Radiation is everywhere, but can be difficult to detect.

6. Becquerel
Becquerel realized uranium emitted seemingly invisible energy.

7. Marie Curie and her husband Pierre named the energy radioactivity.
Radium salts, after being placed on a photographic plate, leave behind the dark traces of radiation.

8. Geiger-Muller Counter
Early discoveries of radiation relied on photographic equipment
Later, more sophisticated devices such as the Geiger-Müller counter
were developed to more
precisely measure
radioactivity.

9. Marie Curie
Discovereries and contributions to the field of radioactivity

10. Isotopes
are different atoms of the same element, with a different number of neutrons in the nucleus.
Changing the # of neutrons changes the mass number
Remember: mass # = # protons + # neutrons
isotopes still have the same:
number of protons / atomic number
element symbol
Iso refers to greek word same
Topos – place: isotopes of a particular element are found on the same place on the periodic table (same atomic number)

11. Isotopes
Examine the three different carbon atoms right. How are they different from one another?
Why are these isotopes all still grouped together as carbon?

12. Isotopes Isotopes have different neutron numbers.
Why are these isotopes all still grouped together as carbon?

13. Atomic number stays the same as proton number doesn’t change.
Neutron number changes = different atomic mass.

14. Atomic Mass (the decimal #’s)
Atomic mass = average of the mass numbers for all isotopes of an element.
If 19.9% of boron atoms have 5 neutrons, 80.1% have 6 neutrons
19.9% have a mass number of 10, and 80.1% have a mass number of 11
(.199 * 10) + (.801*11) = 10.8
atomic mass of boron = 10.8

16. Representing Isotopes
Isotopes are written using standard atomic notation (nuclear symbol).
Chemical symbol + atomic number + mass number.
Potassium has three isotopes,

17. Write the standard atomic symbol for the following:
Potassium-39
Potassium-40
Potassium-41

18. Write the standard atomic symbol for the following:
Potassium-39
Potassium-40
Potassium-41

19. Potassium is found in nature in a certain ratio of isotopes
93.2% is potassium-39
1.0% is potassium-40
6.7% is potassium-41
Atomic mass
= (.932 x 39) + (.001 x 40) + (.067 x 41)
= 39.1

20. Practice
Reading Check p291
Practice Problems p291
Worksheet

21. Radioactive Decay
Radioactivity results from having an unstable nucleus.
When these nuclei lose energy and break apart, decay occurs.
Radioactive decay is the process by which unstable nuclei lose energy by emitting radiation until they are stable.
They release energy until they become stable, often different atoms.
An element may have only certain isotopes that are radioactive. These are called radioisotopes.
Unlike all previously discovered chemical reactions, radioactivity sometimes results in the formation of completely new atoms.
Radioactivity results from having an unstable nucleus.
When these nuclei lose energy and break apart, decay occurs.
Radioactive decay releases energy from the nucleus as radiation.
Radioactive atoms release energy until they become stable, often as different atoms.
An element may have only certain isotopes that are radioactive. These are called radioisotopes.

22. Radioactive Decay
How does the radioisotope Uranium-238 decay?

23. Radioactive Decay
How does the radioisotope Uranium-238 decay? Change Mass #, And Atomic #

24. Three Types of Radiation
Ernest Rutherford ( ) identified three types of radiation using an electric field.
Rutherford's electromagnetic wave detector consisted of a magnetizing coil with iron wire. (electrical field) In 1899, He also named gamma rays, a form of electromagnetic radiation.
Ernest Rutherford postulated the nuclear structure of the atom, discovered alpha and beta rays, and proposed the laws of radioactive decay. He received the Nobel Prize in Chemistry in 1908.
first person to successfully split an atom using nitrogen and alpha particles.
famed Gold Foil Experiment of 1909 demonstrated that atoms were made up of a charged nucleus orbited by electrons. He published his findings in 1911 with a description of what he called the Rutherford model of the atom. While electrons had been described prior to this experiment, Rutherford was the first person to describe the existence of a small, charged atomic nucleus.
See page 294
(c) McGraw Hill Ryerson 2007

25. Positive alpha particles were attracted to the negative plate.
Negative beta particles were attracted to the positive plate.
Neutral gamma rays did not move towards any plate.

26. Alpha Radiation Alpha radiation is a stream of alpha particles.
What are some characteristics of Alpha particles?
See page
Radium-226 releases an alpha particle and becomes
Radon-222. Radon has two less protons than radium.
(c) McGraw Hill Ryerson 2007

27. Alpha Radiation Alpha radiation is a stream of alpha particles.
They are positively charged, and are the most massive of the radiation types.
Alpha particles are essentially the same as helium atoms.
Alpha particles are represented by the symbols
Because it has two protons, it has a charge of 2+.
The release of alpha particles is called alpha decay.
Alpha particles are slow and penetrate materials much less than the other forms of radiation. A sheet of paper will stop an alpha particle.
Produced during radioactive decay
Represented as either 4 2 a or 4 2 He
Made of 2 protons and 2neutrons
2 protons = 2+ charge
Alpha particles are alos slow and don’t penetrate materials very deeply )piece of paper will stop them)
See page
Radium-226 releases an alpha particle and becomes
Radon-222. Radon has two less protons than radium.
(c) McGraw Hill Ryerson 2007

28. Alpha Radiation is a stream of alpha particles
positively charged, and are the most massive of the radiation types.
essentially the same as helium atoms.
represented by the symbols .
Because it has two protons, it has a charge of 2+.
The release of alpha particles is called alpha decay.
slow and penetrate materials much less than the other forms of radiation. A sheet of paper will stop an alpha particle.
Produced during radioactive decay
Represented as either 4 2 a or 4 2 He
Made of 2 protons and 2neutrons
2 protons = 2+ charge
Alpha particles are also slow and don’t penetrate materials very deeply )piece of paper will stop them)
See page
(c) McGraw Hill Ryerson 2007

29. Alpha Radiation Radium-226 releases an alpha particle and becomes
Note how equation is balanced
Pracitce problems p295
Radium-226 releases an alpha particle and becomes
Radon-222. Radon has two less protons than radium.

30. Practice
Practice Problems p295

31. Beta Radiation
What are some characteristics of Beta Particles?

32. Beta Radiation
A beta particle is an electron and is negatively charged.
represented by:
Electrons are very tiny, so beta particles are assigned a mass of 0.
Since there is only an electron, a beta particle has a charge of 1–.
Produced during radioactive decay
Representation …
Made of an e so charge is 1-
Produced when a neutron decays into proton and electron (atomic number doesn’t change but atomic mass does. Neutron is retained and e is released.

33. Beta Radiation
Beta decay occurs when a neutron changes into a proton and an electron.
The proton stays in the nucleus, and the electron is released.
It takes a thin sheet of aluminum foil to stop a beta particle.
Can be stopped by sheet of aluminum
Is the equation balanced? Yes, atomic number and mass are both equal.
Creation of a proton means the atom produced is changed, but mass is same as a neutron has been replaced by a proton of almost equal mass.
Complete practice problems on p296
Iodine-131 releases a beta particle and becomes xenon-131. A neutron has turned into a proton and the released electron.
See page 296
(c) McGraw Hill Ryerson 2007

34. Practice
Do practice problems p296

35. Gamma Radiation What are some characteristics of Gamma particles?

36. Gamma Radiation
Gamma radiation is a ray of high-energy, short-wavelength radiation.
Gamma radiation has no charge and no mass, and is represented by:
Has no charge
highest-energy form of electromagnetic radiation.
It takes thick blocks of lead or concrete to stop gamma rays.
Often, other kinds of radioactive decay will also release gamma radiation.
Uranium-238 decays into an alpha particle and also releases gamma rays.
Produced by release of high-energy, short wavelength radiation from a high-enery nucleus (*)
See page 297
(c) McGraw Hill Ryerson 2007

37. Is the gamma Decay reaction balanced?
Yes, atomic and mass numbers are equal on both sides of arrow.

38. Radiation Summary
Nuclear equations are written like chemical equations, but represent changes in the nucleus of atoms.
Chemical equations represent changes in the position of atoms, not changes to the atoms themselves.
The sum of the mass
numbers does not change.
2. The sum of the charges
in the nucleus does not
change.
See pages
(c) McGraw Hill Ryerson 2007
Take the Section 7.1 Quiz

39. Radioactivity Decay Summary

40. Check your progress How is mass number of an element determined?
How do you represent a larger nucleus such as radium-226?
Why does an alpha particle have a positive charge?
How does beta decay result in the production of an element with one more protons than the nucleus started out with?
Since gamma rays are not made of matter, how can they be detected?
Mas number is the umber of paricels in nucleus (sum of protons + nneutorns)
Nucleus was represented by writing down the number of protons and number of neutrons present
2 of the 4 subatomic particles in an alpha particle are protons, each of which contribute a charge of 1+ or a total charge of 2+ on the alpha particle.
In Beta decay, a neutron decays into a proton, which remain in the nucleus and an electron which is emitted from the nucleus as beta radiation.
Gamma rays can be detect by screen coated with zinc sulfide, which glow when struck by gamma rays. Also, a GM counter can detect gamma rays.

41. Reading Check
Page 297