 Chemical Properties  Determined by ▪ number of electrons ▪ positions of electrons

 Nuclear Properties  Determined by ▪ Size of the nucleus ▪ Ratio between neutrons and protons

 Atomic Nuclei are either  Stable ▪ Vast majority of atoms are stable  Unstable ▪ Source of radiation ▪ Used in Nuclear Reactors ▪ Powers Nuclear Bombs

 Naturally Radioactive Nuclei  Have an neutron/proton ratio that is abnormal  Exist Outside Band of stability ▪ 1 neutron for every 1 proton (up to element 20) ▪ 1.5 neutrons for every 1 proton (elements from #20-#82)  Have a very heavy nucleus ▪ All atoms with an atomic number past 82 are unstable

 Radioactive Isotopes  Have unstable nuclei  Exist outside the band of stability  Emit several different types of radiation ▪ (to become stable nuclei)

 Emission of Radiation is one way for an unstable nucleus to become a stable one.  Nuclear Radiation  Is usually ▪ Alpha ▪ Beta ▪ Gamma

 Nuclear Radiation  Alpha: He +2 charge: helium-4 nuclei, travel low speed, easily stopped  Beta: e -1 charge: penetrate 100 times more than alpha  Gamma: γ no charge, hi speed electromagnetic radiation of very short wavelengths, bursts of energy emitted from nucleus

 More Nuclear Radiation  Positrons: e +1 charge: exact same properties as electron, with positive charge (form of anti-matter)  Electron capture: e e- pulled into nucleus, causes p + to convert into n 0  Neutron bombardment: n 0 fired into nucleus, bumps p + out. (Used in nuclear reactors to start chain reaction)

 All of these rxns began w/ an unstable nucleus which spontaneously emitted some form of radiation.  Conversion of one element to another is called transmutation.  It can also be induced by bombarding a stable nucleus with high energy , ,  or neutrons.

 Radioactive Decay Series: the series of nuclear reactions an unstable atom undergoes to become stable

 Starting atom: U-238  Ending atom: Pb-206  Why did we stop at Pb-206?

 Three most common series:  U-238  Pb-206  U-235  Pb-207  Th-232  Pb-208  Why do they all stop at lead?

 Alpha emission:  radium-226 decays emitting alpha particles Ra  He + Rn

 Alpha emission:  Radon-222 decays emitting alpha particles Ra  He + _______

 Beta emission:  thorium-234 undergoes beta decay: Th  e + Pa

 Beta emission:  lead-214 undergoes beta decay: Pb  e + __________

 Gamma Radiation:  No charge  no particle change to nucleus  no equation needed

 Positron emission:  Phosphorus-32 emits a positron P  e + + ________

 Neutron Bombardment:  Nitrogen-14 is bombarded with neutrons N + n 0  p + + ________

1. alpha decay of Uranium alpha decay of Barium beta decay of Radon beta decay of carbon-11

5. Oxygen-15 undergoing positron emission 6. neutron bombardment of U-235

 The time it takes for half of a radioactive substance to decay  Example:  The half-life of strontium-90 is 28 years. If you started with 10 g of Sr-90, how much remains after 28 years?  56 years?  84 years?  112 years?

 physics/alpha-decay_en.jnlp physics/alpha-decay_en.jnlp

 Radium’s half-life = 1620 yrs.  Starting with 40g of Ra, how much remains in 1620 yrs?  3240 yrs?  4860 yrs?

 The half-life of I-131 is 8 days. If only 1/8 th the original amt of this isotope of I remains, how old is the I-131 ?

 After 3 years only 1/64 th of the original mass of an element remained. What was its half life?

 Carbon in living organisms is a constant ratio of C-14 (radioactive) to C-12 (stable). When organism dies (stops breathing in C-14 from atmosphere), no fresh C-14 replaces the C-14 decaying in its tissues.  C-14 has a half life of 5700 years.

 C-14 dating CANNOT be used to date objects that are more than about 50,000 yrs old.  1. after 8 half-lives the radioactivity is too low to be measured accurately (1/2) 8 = 1/256 th  2. can’t be sure that the ratio of C-14 to C-12 was the same long ago as it is now (depends on solar activity & ocean pH)

 An ancient wooden dish is found to contain 25 % of the amt of C-14 that is in a living tree. About how old is this dish?  A museum mummy contains 1/8 th of the amt of C- 14 as a living Egyptian. How old is the mummy?