Radioactivity

 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 = μ - mass of He nucleus = μ  μ of mass is lost  Mass/energy did not disappear, its gone into another form Binding Energy

 Lost mass becomes energy E = mc 2  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 Total Binding Energy

 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 Radioactivity

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

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

 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 Alpha Decay

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

 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

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

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

 Since the parent loses a charge of –1, the daughter must gain a charge of C  7 14 N + e  Again, the identity of the atom changes – transmutation

Beta Decay

 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)

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

 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 Gamma Decay

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

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

 Time for ½ of sample to decay Half-Life

 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 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? Example