Nuclear Chemistry Notes

Nuclear Chemistry

• The study of the structure of atomic nuclei and the changes they undergo.

Guiding Questions

• Is radiation dangerous?
• Is nuclear power a good choice?
• What is nuclear energy?
• Are nuclear energy and nuclear bombs both dangerous?

Radioactive Decay

• Discovered by Antoine Henri Becquerel in 1896.
• He observed bright spots on photographic plates exposed to uranium metals.
• Placing a uranium compound on a wrapped sheet of film results in an exposed spot on the developed negative (Figure 4.1).

Isotopes and Radioisotopes

• Isotopes: Atoms of the same element with different numbers of neutrons.
• Radioisotopes: Isotopes of atoms with unstable nuclei.

Stable vs. Unstable Isotopes

• Stable Isotopes: Atoms that do not release protons or neutrons from the nucleus and are not radioactive.
• Unstable Isotopes: Atoms that spontaneously release protons and neutrons from their nucleus and are radioactive.

Band of Stability

• The region on a graph indicating all stable nuclei when the number of neutrons is compared to the number of protons.
• Each point on the graph represents a stable atom.
• The neutron-to-proton (n/p) ratio varies within the band of stability.
  • For example:
    • Helium (He): n/p ratio = 1.0
    • Silver (Ag): n/p ratio = 1.28
    • A point on the graph: n/p ratio = 1.51

Chemical vs. Nuclear Reactions

Mass Defect

• Some mass can be converted into energy.
• Described by Einstein's famous equation: E = mc^2
  • Where:
    • E = Energy
    • m = Mass
    • c = Speed of light

Types of Radiation

• The effect of an electric field on three types of radiation:
  • Alpha (α) particles: Positively charged, deflected toward the negatively charged plate.
  • Beta (β) particles
  • Gamma (γ) radiation
• Figure 4.2 illustrates the penetrating power of different types of radiation.
  • Alpha (α) radiation: stopped by paper, metal foil, or thin clothing
  • Beta (β) radiation: stopped by metal sheeting, dense wood, or heavy clothing
  • Gamma (γ) radiation: stopped by thick walls of lead or concrete

Products of Natural Radioactivity

• A stream of particles is sometimes called a ray (e.g., gamma ray).
• Figure 4.4 illustrates the components of alpha, beta, and gamma rays.
  • Alpha particle: High-energy helium nucleus (He^{2+})
  • Beta particle: High-energy electron
  • Gamma radiation: High-energy electromagnetic radiation

Types of Radioactive Decay

• Alpha Particle Emission
  • Loss of a helium nucleus (He).
  • Example:
    • Parent nuclide: {}_{92}^{238}U
    • Daughter nuclide: {}{90}^{234}Th + {}2^4He

Beta Decay

• Nuclear changes that accompany the emission of a beta particle.
  • Example:
    • {}{6}^{14}C rightarrow {}{7}^{14}N + {}_{-1}^{0}e

Beta Particle Emission

• Neutron decay:
  • {}0^1n rightarrow {}1^1p + {}_{-1}^{0}e

Gamma Radiation

• High-energy (short wavelength) electromagnetic radiation, denoted by the symbol γ.
• Emission of gamma rays does not change the atomic number or mass number of a nucleus.
• Gamma rays almost always accompany alpha and beta radiation, accounting for most of the energy loss during nuclear decay.

Induced Nuclear Reactions

• Scientists can force (= induce) nuclear reactions by bombarding nuclei with alpha, beta, and gamma radiation to make the nuclei unstable.

Balancing Nuclear Equations

1. The sums of mass numbers (left superscripts) on each side must be equal.
2. The sums of atomic numbers or nuclear charges (left subscripts) on each side of the equation must be equal.

• Examples:
  • {}{92}^{238}U rightarrow {}2^4He + {}_{90}^{234}Th
  • {}{82}^{214}Pb rightarrow {}{-1}^{0}β + {}_{83}^{214}Bi

Balancing Nuclear Equations - Practice

• Complete the following nuclear equations:
  1. {}{85}^{217}At rightarrow {}{83}^{213}Bi + ?
     • Solution: {}{85}^{217}At rightarrow {}{83}^{213}Bi + {}_2^4He
  2. {}{90}^{231}Th rightarrow {}{-1}^{0}β + ?
     • Solution: {}{90}^{231}Th rightarrow {}{-1}^{0}β + {}_{91}^{231}Pa
  3. {}{81}^{208}Tl rightarrow {}{-1}^{0}β + ?
     • Solution: {}{81}^{208}Tl rightarrow {}{-1}^{0}β + {}_{82}^{208}Pb

Nuclear Reactions - Fission vs. Fusion

• Two types:
  • Fission: The splitting of nuclei.
  • Fusion: The joining of nuclei.
• Both reactions involve extremely large amounts of energy.
• Albert Einstein’s equation E = mc^2 illustrates the energy found in even small amounts of matter.

Nuclear Fission

• The splitting of one heavy nucleus into two or more smaller nuclei, as well as some sub-atomic particles and energy.
• A heavy nucleus is usually unstable due to many positive protons pushing apart.
• When fission occurs:
  • Energy is produced.
  • More neutrons are given off.

Nuclear Fission (Continued)

• Neutrons are used to make nuclei unstable.
• It is much easier to crash a neutral neutron than a positive proton into a nucleus to release energy.

Nuclear Fission - Practice

• Complete the following nuclear equations:
  • (a) {}{92}^{238}U + {}0^1n
    rightarrow {}_{92}^{239}U + ?
  • (b) {}{4}^{9}Be + {}1^1H
    rightarrow {}_{3}^{6}Li + ?
  • (c) {}{4}^{9}Be + {}2^4He
    rightarrow {}_{6}^{12}C + ?

Nuclear Fission – Chain Reaction

• Fission produces a chain reaction.

Nuclear Fusion

• Joining of two light nuclei into one heavier nucleus.
  • In the core of the Sun, two hydrogen nuclei join under tremendous heat and pressure to form a helium nucleus.
  • When the helium atom is formed, huge amounts of energy are released.

Nuclear Fusion Challenges

• Scientists cannot yet find a safe and manageable method to harness the energy of nuclear fusion.
  • "Cold fusion" would occur at temperatures and pressures that could be controlled (but we haven’t figured out how to get it to happen).

Nuclear Fusion - Practice

• Complete the following nuclear equations, thought to be the source of the energy of some stars:
  • (a) {}1^1H + {}{6}^{12}C
    rightarrow ?
  • (b) {}{7}^{13}N rightarrow {}{6}^{13}C + ?
  • (c) {}{6}^{13}C + {}1^1H
    rightarrow ?
  • (d) {}1^1H + {}{7}^{14}N
    rightarrow ?
  • (e) {}{8}^{15}O rightarrow {}{7}^{15}N + ?
  • (f) {}{7}^{15}N + {}1^1H
    rightarrow {}_{6}^{12}C + ?

Applications of Nuclear Chemistry

• Medicine
  • Chemotherapy
  • Power pacemakers
  • Diagnostic tracers
• Agriculture
  • Irradiate food
  • Pesticide
• Energy
  • Fission
  • Fusion

Food Irradiation

• Food can be irradiated with γ rays from ^{60}Co or ^{137}Cs.
• Irradiated milk has a shelf life of 3 months without refrigeration.
• USDA has approved irradiation of meats and eggs.

Challenges of Nuclear Power

• Disposal of waste products.

Nuclear Waste Disposal

• Hazardous wastes produced by nuclear reactions are problematic.
  • Some waste products, like fuel rods, can be re-used.
  • Some products are very radioactive and must be stored away from living things.
• Most of this waste is buried underground or stored in concrete.
• It takes 20 half-lives (thousands of years) before the material is safe.

Albert Einstein

• Discovered the equation that relates mass and energy: E=mc^2