Radioactivity

1. Radioactivity

2. Binding Energy Total mass of nucleus is always less than the sum of its protons and neutrons Compare the mass of He-4 to that of its nucleus - mass of 2 protons + 2 neutrons = 4.031882 μ - mass of He nucleus = 4.002602 μ 0.02928 μ of mass is lost Mass/energy did not disappear, its gone into another form

3. Total Binding Energy Lost mass becomes energy E = mc2 Energy from the mass is being used to hold the nucleus together – Called Binding Energy The nucleus is composed of protons (+) and neutrons (neutral), so electrostatic forces are pushing the particles apart Binding energy forms the Strong Nuclear Force holding the nucleus together

4. Radioactivity Emission of subatomic particles or high energy electromagnetic radiation from the nucleus of an atom Atoms with a higher neutron to proton ratio will decay spontaneously

6. Discovery Discovered in 1896 by Henri Becquerel Marie and Pierre Curie discovered radioactive elements and emissions produced by decay

7. 3 Types of Radioactivity Alpha Decay () Beta Decay () Gamma decay ()

8. Alpha Decay Atom decays into a smaller atom and a positively charged alpha particle () Alpha particle consists of a He nucleus (2 protons, 2 neutrons) Atom undergoing alpha decay is called the parent When the parent emits the  particle, it becomes the daughter

9. Process of forming the new element is called transmutation 226Ra  222Rn + 4He

11. Has largest ionizing power = ability to ionize molecules & atoms due to largeness of -particle But has lowest penetrating power = ability to penetrate matter Skin, even air, protect against -particle radiation

12. Example U – 235 undergoes alpha decay. Write the nuclear equation.

13. Beta Decay Emission of the electron from a nucleus Electron is emitted from the nucleus as a neutron changes into a proton 10n  11p + 0-1e No nucleons are lost, therefore the atomic mass is the same for the parent and daughter

14. Since the parent loses a charge of –1, the daughter must gain a charge of +1 614C  714N + e Again, the identity of the atom changes – transmutation

15. Beta Decay

16. Lower ionizing power than alpha particle But higher penetration power Requires sheet of metal or thick piece of wood to arrest penetration  more damage outside of body, but less in (alpha particle is opposite)

17. Example K – 43 undergoes beta decay, write the nuclear reaction.

18. Gamma Decay Very similar to emission of photons by excited electrons Nucleus itself is excited and when it falls back to lower energy, emits a chargeless photon which is gamma radiation Gamma decay does not change the element Happens along with alpha and beta decay

19. Lowest ionizing power, highest penetrating power  requires several inches lead shielding

20. Practice Write a nuclear equation for each of the following: 1. beta decay in Bk-249 2. alpha decay of Ra-224

21. Half-Life Time for ½ of sample to decay

22. Example P-32 has a half-life of 14.3 days. If you start with 4.0 mg of P-32, how many mg remain after 57.2 days? The half-life of radon is 3.824 days. After what time will 0ne-fourth of the given amount of radon remain? A sample begins with 16mg of Po. After 12 min, the sample contains only 1.0mg. What is the half-life of Po?