Chapter 13 Nuclear Chemistry

Nuclear radioactivity Natural radioactivity Radioactivity - emission of particles or energy from an unstable nucleus Discovered by Henri Bequerel Three types of radioactive decay Alpha decay (He-nucleus) Beta decay (high energy electron) Gamma decay (high energy electromagnetic waves)

The nature of the nucleus Strong nuclear force Binds protons and neutrons Very short ranged, less than 10-15 m Overcomes proton-proton Coulomb repulsion Band of stability

Generalizations - nuclear stability Atomic number > 83: unstable Nucleon number = 2, 8, 20, 28, 50, 82 or 126: added stability Pairs of protons and pairs of neutrons: added stability Odd number of both protons and neutrons less stable Neutron: proton ratios for added stability 1:1 in isotopes with up to 20 protons 1+increasing:1 with increasingly heavy isotopes

Nuclear equations Atomic number = number of protons in nucleus Isotopes: same atomic number; different number of neutrons Mass number = number of nucleons (protons and neutrons) in nucleus Nuclear reactions Represented by balanced equations Charge conserved Atomic number conserved

Types of radioactive decay Alpha emission Expulsion of helium nucleus Least penetrating: stopped by paper Beta emission Expulsion of an electron More penetrating: 1 cm of aluminum Gamma decay Emission of a high energy photon Most penetrating: 5 cm of lead

Radioactive decay series One radioactive nucleus decays to a 2nd, which decays to a 3rd, which… Three naturally occurring series Thorium-232 to lead-208 Uranium-235 to lead-207 Uranium-238 to lead-206 Process results in a stable nucleus

Writing Nuclear Equations Determine what type of decay occurs Write what you start with on the left side of the arrow Write the decay particle emitted on the far right Using conservation of mass and atomic #, determine what the new nucleus will be

Things to remember: Alpha decay gives off a He nucleus Remaining nucleus has a mass 4 lower and atomic # 2 lower than original Beta decay gives off a high energy e- Remaining nucleus has the same mass and an atomic # 1 higher than original

Practice Thorium-232 undergoes Alpha decay Radium 228 undergoes Beta decay

Radioactive Decay Rates It is impossible to predict the exact moment a nucleus will decay We CAN predict the time for half the nuclei in a sample to decay

Half-life Time required for 1/2 of a radioactive sample to decay Example: 1 kg of an unstable isotope with a one-day half-life After 1 day: 500 g remain After 2 days: 250 g remain After 3 days: 125 g remain U-238 decay series: wide half-life variation

Calculating Half Lives Radium-226 has a half life of 1599 years. How long would it take seven-eighths of a radium-226 sample to decay? Step 1- List the given and unknown values Given: half-life= 1599 years; fraction of sample decayed=7/8 Unknown: fraction of sample remaining?; total time of decay?

Calculating Half Lives Step 2- Calculate the fraction of radioactive sample remaining Subtract fraction decayed from 1 1-7/8= 1/8 remaining Step 3- Calculate the number of half lives Amount remaining after 1 half-life= ½ After 2 half-lives=1/4 After 3 half-lives=1/8

Calculating Half-Lives Radium-226 has a half life of 1599 years. How long would it take seven-eighths of a radium-226 sample to decay? Step 4- Calculate the total time required for the radioactive decay 3 half-lives x 1599 years=4797 years

Nuclear energy There is an overall loss of mass in nuclear reactions Why? Interconversion of mass and energy Change in mass is related to a change in energy Mass defect Difference between masses of reactants and products

Nuclear Energy Binding energy Energy out? Energy required to break a nucleus into individual protons and neutrons Energy out? E=mc2 E= energy in J M= mass defect in kg C= speed of light 3.0e8

Nuclear fission Heavy nuclei splitting into lighter ones Chain reactions Possible when one reaction can lead to others One neutron in, two or more out Critical mass Sufficient mass and concentration to produce a chain reaction

Nuclear power plants Rely on controlled fission chain reactions Steel vessel contains fuel rods and control rods Full plant very intricate Containment and auxiliary buildings necessary Spent fuel rods Contain fissionable materials U-235, Pu-239 Disposal issues not settled

Many possible fission fragments

Nuclear fusion Less massive nuclei forming more massive nuclei Energy source for Sun and other stars Requirements for fusion High temperature High density Sufficient confinement time

Source of nuclear energy Ultimately connected to origins of the Universe and the life cycles of stars Big Bang theory Incredibly hot, dense primordial plasma cools, creating protons and neutrons Continued cooling leads to hydrogen atoms which collapse gravitationally into 1st generation stars Stellar evolution Interior temperatures and densities suitable for fusion of heavy elements beyond hydrogen and helium Certain massive stars explode in supernovae, spreading heavy elements (some radioactive) Ultimate source: gravitational attraction!