NUCLEAR CHEMISTRY Introduction to Nuclear Chemistry Nuclear chemistry is the study of the structure of atomic nuclei and the changes they undergo. Chemical vs. Nuclear Reactions Chemical Reactions Nuclear Reactions Occur when bonds are broken Occur when nuclei emit particles and/or rays Chemical vs. Nuclear Reactions Chemical Reactions Nuclear Reactions Occur when bonds are broken Occur when nuclei emit particles and/or rays Atoms remain Atoms often unchanged, although converted into atoms they may be of another element rearranged Chemical vs. Nuclear Reactions Chemical Reactions Nuclear Reactions Occur when bonds are broken Occur when nuclei emit particles and/or rays Atoms remain unchanged, although they may be rearranged Atoms often converted into atoms of another element Involve only valence May involve protons, electrons neutrons, and electrons Chemical vs. Nuclear Reactions Chemical Reactions Nuclear Reactions Occur when bonds are broken Occur when nuclei emit particles and/or rays Atoms remain unchanged, although they may be rearranged Atoms often converted into atoms of another element Involve only valence electrons May involve protons, neutrons, and electrons Associated with small Associated with energy changes large energy changes Chemical vs. Nuclear Reactions Chemical Reactions Nuclear Reactions Occur when bonds are broken Occur when nuclei emit particles and/or rays Atoms remain unchanged, although they may be rearranged Atoms often converted into atoms of another element Involve only valence electrons May involve protons, neutrons, and electrons Associated with small energy changes Associated with large energy changes Reaction rate influenced by temperature, particle size, concentration, etc. Reaction rate is not influenced by temperature, particle size, concentration, etc. The Discovery of Radioactivity (1895 – 1898): Roentgen found that invisible rays were emitted when electrons bombarded the surface of certain materials. Becquerel accidently discovered that phosphorescent uranium salts produced spontaneous emissions that darkened photographic plates The Discovery of Radioactivity (1895 – 1898): Marie Curie isolated the components (uranium atoms) emitting the rays Identified 2 new elements, polonium and radium, on the basis of their radioactivity These findings contradicted Dalton’s theory of indivisible atoms. Radioactivity – process by which particles give off rays Radiation – the penetrating rays and particles emitted by a radioactive source The Discovery of Radioactivity (1895 – 1898): Isotopes – atoms of the same element with different numbers of neutrons Radioisotopes – isotopes of atoms with unstable nuclei (too many/few neutrons) Radioactive decay – when unstable nuclei lose energy by emitting radiation to attain more stable atomic configurations (spontaneous process) Chemical Symbols A chemical symbol looks like… 14 6 C To find the number of neutrons, subtract the atomic # from the mass #. Alpha radiation Composition – Alpha particles, same as helium nuclei 4 Symbol – Helium nuclei, 2He, α Charge – 2+ Mass (amu) – 4 Approximate energy – 5 MeV Penetrating power – low (0.05 mm body tissue) Shielding – paper, clothing Beta - radiation Composition – Beta particles, same as an electron Origin – neutron turned to a proton + electron Symbol – e-, βCharge – 1Mass (amu) – 1/1837 (practically 0) Approximate energy – 0.05 – 1 MeV Penetrating power – moderate (4 mm body tissue) Shielding – metal foil Beta + radiation Composition – Beta particles, exact opposite of an electron, called a positron Origin – proton turned to a neutron + positron Symbol – e+, β+ Charge – 1+ Mass (amu) – 1/1837 (practically 0) Approximate energy – 0.05 – 1 MeV Penetrating power – moderate (4 mm body tissue) Shielding – metal foil Gamma radiation Composition – High-energy electromagnetic radiation Symbol – γ Charge – 0 Mass (amu) – 0 Approximate energy – 1 MeV Penetrating power – high (penetrates body easily) Shielding – lead, concrete Nuclear Stability Isotope is completely stable if the nucleus will not spontaneously decompose. Elements with atomic #s 1 to 20 are very stable. 1:1 ratio of protons : neutrons (p+: n0) Example: Carbon – 12 has 6 protons and 6 neutrons Nuclear Stability Elements with atomic #s 21 to 82 are marginally stable. 1:1.5 ratio of protons:neutrons (p+ : n0) Example: Mercury – 200 has 80 protons and 120 neutrons Nuclear Stability Elements with atomic #s >82 are unstable and radioactive. Examples: Uranium-235 and plutonium-239 Alpha Decay Alpha decay – emission of an alpha particle (α), denoted by the symbol 42 Alpha decay causes the mass # to decrease by 4 and the atomic # to decrease by 2. Atomic # determines the element. All nuclear equations are balanced with respect to mass #s AND atomic #s. Alpha Decay Example 1: Write the nuclear equation for the radioactive decay of polonium – 210 by alpha emission. Step 4: 1: Determine 2: 3: Draw the Write the arrow. element alpha the other particle. that product you are (ensuring starting with. everything is balanced). Mass # Atomic # Alpha Decay Example 2: Write the nuclear equation for the radioactive decay of radium – 226 by alpha emission. Step 4: 1: Determine 2: 3: Draw the Write the arrow. element alpha the other particle. that product you are (ensuring starting with. everything is balanced). Mass # Atomic # Beta - decay Beta decay – emission of a beta particle (β-), a fast moving electron, denoted by the symbol e- or-10 . Beta decay causes no change in mass number and causes the atomic number to increase by 1. Beta- Decay Example 1: Write the nuclear equation for the radioactive decay of carbon – 14 by betaemission. Step 4: 1: Determine 2: 3: Draw the Write the arrow. element beta the other particle. that product you are (ensuring starting with. everything is balanced). Mass # Atomic # Beta- Decay Example 2: Write the nuclear equation for the radioactive decay of zirconium – 97 by beta decay. Step 4: 1: Determine 2: 3: Draw the Write the arrow. element beta the other particle. that product you are (ensuring starting with. everything is balanced). Mass # Atomic # Beta + decay Beta+ decay – emission of a beta particle (β+), a fast 0 moving positron, denoted by the symbol e+ or+1 . Beta+ decay causes no change in mass number and causes the atomic number to decrease by 1. Beta + Decay Example 1: Write the nuclear equation for the radioactive decay of calcium – 37 by positron emission. Step 4: 1: Determine 2: 3: Draw the Write the arrow. element beta the other particle. that product you are (ensuring starting with. everything is balanced). Mass # Atomic # Beta+ Decay Example 2: Write the nuclear equation for the radioactive decay of potassium – 37 by beta+ decay. Step 4: 1: Determine 2: 3: Draw the Write the arrow. element beta the other particle. that product you are (ensuring starting with. everything is balanced). Mass # Atomic # Gamma decay Gamma rays – high-energy electromagnetic radiation, denoted by the symbol γ. γ has no mass (0) and no charge (0). Gamma rays almost always accompany alpha and beta radiation. However, since there is no effect on mass number or atomic number, they are usually omitted from nuclear equations. Transmutation Transmutation – the conversion of one atom of one element to an atom of a different element Spontaneous radioactive decay is one way that this occurs (fission) Nuclear fusion in the sun Artificial transmutation by people with particle accelerators to create bigger atoms Typically involve neutrons ( 1 n) as reactants or products 0 Review Type of Radioactive Decay Alpha Beta Beta + Gamma Particle Emitted 4 2 He 0 -1 e 0 +1 e α ββ+ γ Change in Change in Mass # Atomic # -4 0 0 0 -2 +1 -1 0