Ch05. Radiation. Energy and matter that comes from the nucleus of an atom. version 1.6

Transcription

1 Ch05 Radiation Energy and matter that comes from the nucleus of an atom. version 1.6 Nick DeMello, PhD

2 Ch05 Radiation The Discovery of Radioactivity Phosphorescence Radioactive history Antoine Becquerel Marie Curie Ernest Rutherford Properties of Radiation Classes of Radioactivity Alpha Radiation Beta Radiation Gamma Radiation Nuclear Reactions Nuclear Decay Half Life Nuclear Fusion Nuclear Fission

3 Discovery of Radioactivity Antoine-Henri Becquerel designed an experiment to determine if phosphorescent minerals also gave off X-rays. Phosphorescence is the long-lived emission of light by atoms or molecules that sometimes occurs after they absorb light. Antoine Becquerel X-rays are detected by their ability to penetrate matter and expose a photographic plate. 3

4 Discovery of Radioactivity Becquerel discovered that certain minerals were constantly producing energy rays that could penetrate matter. Becquerel determined that all the minerals that produced these rays contained uranium, and the rays were produced even though the mineral was not exposed to outside energy. He called them uranic rays because they were emitted from minerals that contained uranium. Like X-rays Not related to phosphorescence Antoine Becquerel 4

5 Marie Curie Discovered Radioactive Elements Marie Curie determined the rays were emitted from specific elements. She also discovered new elements by detecting their rays. Radium named for its green phosphorescence Polonium named for her homeland Poland Marie Curie Because these rays were no longer just a property of uranium, she renamed it radioactivity. 5

6 Properties of Radioactive Rays Radioactivity is the release of tiny, high-energy particles or energy from an atom. Nuclear Radioactivity is release from the nucleus of an atom. Radioactive rays can ionize matter. Cause uncharged matter to become charged That is how a Geiger counter and electroscope function. Radioactive rays have high energy. Radioactive rays can penetrate matter. Radioactive rays cause phosphorescent chemicals to glow. That how a scintillation counter works. 6

7 Ch05 Radiation The Discovery of Radioactivity Phosphorescence Radioactive history Antoine Becquerel Marie Curie Ernest Rutherford Properties of Radiation Classes of Radioactivity Alpha Radiation Beta Radiation Gamma Radiation Nuclear Reactions Nuclear Decay Half Life Nuclear Fusion Nuclear Fission

8 Radiation & Radioactivity Ernest Rutherford discovered three forms of emissions that come from radioactive elements. Gamma rays have no charge and and have no mass. Beta particles have a negative charge, beta radiation like cathode rays, are a stream of electrons. Alpha particles have a positive charge and as much mass as a helium atom. (four times the mass of a hydrogen atom) Rutherford Mass Charge α particles A Helium atom positive β particles Electrons negative γ rays none none β γ α

9 Radiation & Radioactivity Ernest Rutherford discovered three forms of emissions that come from radioactive elements. Gamma rays have no charge and and have no mass. Beta particles have a negative charge, beta radiation like cathode rays, are a stream of electrons. Alpha particles have a positive charge and as much mass as a helium atom. (four times the mass of a hydrogen atom) Rutherford Mass Charge α particles A Helium atom positive Pieces of Lead β particles Electrons negative γ rays none none α β γ mm 1 mm 100 mm

10 Radioisotopes It s about mass. Only certain isotopes of a given element are radioactive. Carbon-14 is radioactive, Carbon-12 is not. Hydrogen-3 is radioactive, Hydrogen-1 is not. Silicon-27 ( 27 Si) is radioactive, Silicon-28 ( 28 Si) is not. All the isotopes of Polonium are radioactive ( 208 Po, 209 Po, 210 Po). None of the isotopes of copper are radioactive. Radioisotopes are those isotopes of an element that are radioactive. 10

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12 Ch05 Radiation The Discovery of Radioactivity Phosphorescence Radioactive history Antoine Becquerel Marie Curie Ernest Rutherford Properties of Radiation Classes of Radioactivity Alpha Radiation Beta Radiation Gamma Radiation Nuclear Reactions Nuclear Decay Half Life Nuclear Fusion Nuclear Fission

13 Nuclear Reactions Chemical reactions involve changes in the electronic structure of the atom. Atoms gain, lose, or share electrons. No change in the nuclei occurs. Gold will always be gold in a chemical reaction. Nuclear reactions involve changes in the structure of the nucleus. When the number of protons in the nucleus changes. The atom becomes a different element. In a nuclear reaction, Lead can become Gold. 13

14 Nuclear Reactions We describe nuclear processes with nuclear equations. Atomic numbers and mass numbers are conserved. The sum of the atomic numbers on both sides must be equal. The sum of the mass numbers on both sides must be equal. You cannot create or destroy matter. 14

15 Nuclear Decay Radioisotopes are not stable. They will degrade, or decay, to form stable isotopes. There are two primary ways a nucleus changes. Alpha decay Loss of an alpha particle. Beta decay A neutron breaking into a proton and electron. Some nuclear events will also release a proton from the nucleus. This proton (or packet of energy) is called gamma radiation. 15

16 Alpha Decay Occurs when an unstable nucleus emits a particle composed of two protons and two neutrons Most ionizing, but least penetrating of the types of radioactivity Loss of an alpha particle means the atomic number decreases by 2, and the mass number decreases by 4. 16

17 Beta Decay Occurs when an unstable nucleus emits an electron, turning a neutron into a proton. About 10 times more penetrating than α, but only about half the ionizing ability When an atom loses a β particle (an electron) its atomic number increases by 1, and the mass number remains the same. 17

18 Gamma Emission Gamma (γ) rays are high energy photons of light. No loss of particles from the nucleus No change in the composition of the nucleus Same atomic number and mass number Least ionizing, but most penetrating Generally occurs after the nucleus undergoes some other type of decay and the remaining particles rearrange 18

19 19 Nuclear Decay

20 Ch05 Radiation The Discovery of Radioactivity Phosphorescence Radioactive history Antoine Becquerel Marie Curie Ernest Rutherford Properties of Radiation Classes of Radioactivity Alpha Radiation Beta Radiation Gamma Radiation Nuclear Reactions Nuclear Decay Half Life Nuclear Fusion Nuclear Fission

21 Rate of Radioactive Decay The rate of change in the amount of radioactivity is constant, and is different for each radioactive isotope. Change in radioactivity measured with Geiger counter Counts per minute Each radionuclide had a particular length of time it required to lose half its radioactivity a constant halflife. We know that processes with a constant half-life follow first order kinetic rate laws. The rate of radioactive change was not affected by temperature. In other words, radioactivity is not a chemical reaction! 21

22 Half-Lives of Various Nuclides We measure the rate of nuclear decay by half lives. A half live is how long it takes an isotope to decay to half it s current mass. Some half lives are very long (thousands of years). Some are very short (fractions of a second). Rates of nuclear decay are very precise. They are so precise, so precise we use them as our standard for time. The decay of Cs isotopes is the definition of a second. The decay of C isotopes is how we carbon date fossils. The decay of U isotopes is how we determined the age of the earth is between 4.0 and 4.5 billion years old. 22

23 Ch05 Radiation The Discovery of Radioactivity Phosphorescence Radioactive history Antoine Becquerel Marie Curie Ernest Rutherford Properties of Radiation Classes of Radioactivity Alpha Radiation Beta Radiation Gamma Radiation Nuclear Reactions Nuclear Decay Half Life Nuclear Fusion Nuclear Fission

24 Fission and Fusion Reactions Fission The large nucleus splits into two smaller nuclei. Fusion Small nuclei can be accelerated to smash together to make a larger nucleus. Both fission and fusion release enormous amounts of energy. Fusion releases more energy per gram than fission. 24

25 Fission and Fusion Reactions Fusion Small nuclei can be accelerated to smash together to make a larger nucleus. All life on earth is fueled by the nuclear fusion going on in our sun. 25

26 Fission and Fusion Reactions Fission The large nucleus splits into two smaller nuclei. Fission (other than alpha decay) is rare in nature. 26

27 Nuclear Chain Reactions Fission reactions can create a chain reaction. Where one fission reaction initiates two more. Which initiate too more, and so on 27

28 Ch05 Radiation The Discovery of Radioactivity Phosphorescence Radioactive history Antoine Becquerel Marie Curie Ernest Rutherford Properties of Radiation Classes of Radioactivity Alpha Radiation Beta Radiation Gamma Radiation Nuclear Reactions Nuclear Decay Half Life Nuclear Fusion Nuclear Fission

29 Questions? 29

30 This semester we will discuss 1. Chemistry Defined the science of matter. 2. Measurement the edge of knowledge. 3. Matter & Energy differences in substances. 4. Atoms & the Elements flavors of the atom. 6. Compounds & Molecules complex particles. Exam #1 Exam #2 7. Reactivity & Stoichiometry transmutation & how much. 8. Chemistry in the Gas State chemistry taught man to fly. 9. Solutions electrolytes & concentration. 10. Acids & Bases moving raw protons. 5. Radioactivity radiation & nuclear reactions. Final Exam Exam #3 The final is cumulative!