NUCLEAR CHEMISTRY

Nuclear Radiation Nuclear chemistry is the study of the structure of atomic nuclei and the changes they undergo.

The Discovery of Radioactivity Marie Curie named the process by which materials such as uranium give off rays radioactivity; the rays and particles emitted by a radioactive source are called radiation.

Types of Radiation As you may recall, isotopes are atoms of the same element that have different numbers of neutrons. Isotopes of atoms with unstable nuclei are called radioisotopes.

Types of Radiation These unstable nuclei emit radiation to attain more stable atomic configurations in a process called radioactive decay. During radioactive decay, unstable atoms lose energy by emitting one of several types of radiation.

Types of Radiation The three most common types of radiation are alpha (α), beta (β), and gamma (γ). Alpha, Beta or Gamma? ?? ?? ??

Types of Radiation Negatively charged beta particles are deflected toward the positively charged plate. ?? ?? ??

Types of Radiation Positively charged alpha particles are deflected toward the negatively charged plate. Alpha particles are deflected less than beta rays because they are more massive. ?? ??

Types of Radiation Gamma rays, which have no electrical charge, are not deflected. ??

Alpha An alpha particle (α) has the same composition as a helium nucleus—two protons and two neutrons—and is therefore given the symbol . The charge of an alpha particle is 2+ due to the presence of the two protons.

Alpha Because of their mass and charge, alpha particles are relatively slow-moving compared with other types of radiation. Thus, alpha particles are not very penetrating—a single sheet of paper stops alpha particles.

Beta A beta particle is a very-fast moving electron that has been emitted from a neutron of an unstable nucleus. Beta particles are represented by the symbol The zero superscript indicates the insignificant mass of an electron in comparison with the mass of a nucleus.

Beta The –1 subscript denotes the negative charge of the particle. Beta radiation consists of a stream of fast-moving electrons.

Beta Because beta particles are both lightweight and fast moving, they have greater penetrating power than alpha particles. A thin metal foil is required to stop beta particles.

Gamma Gamma rays are high-energy (short wavelength) electromagnetic radiation. They are denoted by the symbol . As you can see from the symbol, both the subscript and superscript are zero.

Gamma Thus, the 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, as they account for most of the energy loss that occurs as a nucleus decays.

Types of Radiation He α β e γ 4 +2 -1 Name Symbol Formula Mass Charge Description 4 helium nuclei He 4 +2 alpha α 2 high speed electrons β e beta -1 -1 high energy radiation γ gamma

Nuclear Stability Radioactive nuclei undergo decay in order to gain stability. All elements with atomic numbers greater than 83 are radioactive.

Balancing a Nuclear Equation Nuclear equations are used to show nuclear transformations.  Balanced nuclear equations require that both the atomic number and the mass number must be balanced.

X X Balancing a Nuclear Equation A A Z Z Mass number Element symbol Atomic number Element symbol

beta gamma particle ray Anatomy of a Decay Equation In the early solar system, I-129 underwent beta decay to produce Xe-129. The balanced decay equation is represented: beta gamma particle ray sum of mass numbers must be equal 129 129 I → Xe + e + γ 53 54 -1 reaction arrow reactants products sum of atomic numbers must be equal NO SUBTRACTION

Anatomy of a Decay Equation When writing a sentence as a decay equation “bombard” means the decay particle is on the reactants (left) side. While “decay” means the decay particle is on the products (right) side. alpha bombardment: X + He → Y beta decay: X → Y + e 4 2 0 -1

Anatomy of a Decay Equation Gamma rays are associated with most decay events, but sometimes are not explicitly included in the equation, since they do not affect mass or charge. However it is important to remember that the ENERGY associated with decay is primarily in the form of a gamma rays.

Balancing a Nuclear Equation When beryllium-9 is bombarded with alpha particles (helium nuclei), a neutron is produced.  The balanced nuclear reaction is given as: 9 4 1 Be + He → n + ??? 4 2

Balancing a Nuclear Equation 9 4 1 Be + He → n + ??? 4 2 On the reactant side, the mass numbers equal (9 + 4) = 13. On the product side, the mass number equals 1. The product side needs an additional 12 for the mass number.

Balancing a Nuclear Equation 9 4 1 Be + He → n + ??? 4 2 On the reactant side, the atomic numbers equal (4 + 2) = 6. On the product side, the atomic number equals 0. The product side needs an additional 6 for the atomic number.

Balancing a Nuclear Equation 9 4 1 12 Be + He → n + ??? 4 2 6 The atomic number (the number on the bottom) determines the identity of the element.

Balancing a Nuclear Equation 9 4 1 12 Be + He → n + C 4 2 6 The element with an atomic number of 6 is carbon.

Balancing a Nuclear Equation When nitrogen-14 is bombarded with a neutron, a proton is produced. The balanced nuclear equation can be written as: N + n → p + 14 7 1 ???

Balancing a Nuclear Equation N + n → p + 14 7 1 ??? On the reactant side, the mass numbers equal (14 + 1) = 15. On the product side, the mass number equals 1. The product side needs an additional 14 for the mass number.

Balancing a Nuclear Equation N + n → p + 14 7 1 ??? On the reactant side, the atomic numbers equal (7 + 0) = 7. On the product side, the atomic number equals 1. The product side needs an additional 6 for the atomic number.

Balancing a Nuclear Equation 14 1 1 14 N + n → p + ??? 7 1 6 The atomic number (the number on the bottom) determines the identity of the element.

Balancing a Nuclear Equation 14 1 1 14 N + n → p + C 7 1 6 The element with an atomic number of 6 is carbon.

Balancing a Nuclear Equation 226 230 4 Th → + He ??? Ra 90 88 2 Balance the nuclear reaction above.

Balancing a Nuclear Equation 230 234 4 U → + He ??? Th 92 90 2 Balance the nuclear reaction above.

Balancing a Nuclear Equation 50 50 Co → + e ??? Ni 27 28 -1 Balance the nuclear reaction above.

Balancing a Nuclear Equation Sometimes you must use atoms other than those on the periodic table to balance nuclear reactions.

Balancing a Nuclear Equation

Question What element is formed when undergoes beta decay? Give the atomic number and mass number of the element.

Question Write a balanced nuclear equation for the alpha decay of the following radioisotope.

Question Uranium-238 is bombarded with a neutron. One product forms along with gamma radiation. Write the balanced nuclear equation. U + n → U + γ 238 92 1 239

Question Nitrogen-14 is bombarded with deuterium (hydrogen-2). One product forms along with an alpha particle. Write the balanced nuclear equation. N + H → C + He 14 7 2 1 12 6 4

Radioactive Decay Rates Radioactive decay rates are measured in half-lives. A half-life is the time required for one- half of a radioisotope’s nuclei to decay into its products.

Radioactive Decay Rates For example, the half-life of the radioisotope strontium-90 is 29 years. If you had 10.0 g of strontium-90 today, 29 years from now you would have 5.0 g left. The decay continues until negligible strontium-90 remains.

Radioactive Decay Rates The graph shows the percent of a stontium- 90 sample remaining over a period of four half-lives. With the passing of each half-life, half of the strontium-90 sample decays.

Calculating Amount of Remaining Isotope total time time of 1 half-life number of half-lives n mf = mi ∙ (½) final mass initial mass

Calculating Amount of Remaining Isotope Iron-59 is used in medicine to diagnose blood circulation disorders. The half-life of iron-59 is 44.5 days. How much of a 2.000-mg sample will remain after 133.5 days? (0.2500 mg)

Question Cobalt-60 has a half-life of 5.27 years. How much of a 10.0 g sample will remain after 21.08 years? (0.625 g)

Question Carbon-14 has a half-life of 5730 years. If 125 grams remain after 5730 years, what was the mass of the initial sample?? (250 g)

Question A 40 g sample of Ba-126 decays to 10 g in 200 minutes. What is the half-life (in minutes) of Ba-126? (100 minutes)

Nuclear Fission Heavy atoms (mass number > 60) tend to break into smaller atoms, thereby increasing their stability. Using a neutron to split a nucleus into fragments is called nuclear fission. Nuclear fission releases a large amount of energy and several neutrons.

Nuclear Fission Since neutrons are products, one fission reaction can lead to more fission reactions, a process called a chain reaction. A chain reaction can occur only if the starting material has enough mass to sustain a chain reaction; this amount is called critical mass.

Nuclear Fusion The combining of atomic nuclei is called nuclear fusion. For example, nuclear fusion occurs within the Sun, where hydrogen atoms fuse to form helium atoms.

Nuclear Fusion Fusion reactions can release very large amounts of energy but require extremely high temperatures. For this reason, they are also called thermonuclear reactions.

Comparing Nuclear Reactions Type Description Applications Decay Happens spontaneously, independent of temperature Medicine, Radio-dating Fission Because of chain reactions, creates more energy than decay Nuclear Energy, First atomic bombs Fusion Requires a massive amount of heat and pressure, beyond what can be created (so far) on Earth. Happens in the cores of stars. Thermonuclear weapons, Holy Grail of sustainable energy

Question What is the difference between nuclear fusion and nuclear fission? Nuclear fusion is the combining of nuclei to form a single nucleus. Nuclear fission is the splitting of a nucleus into fragments.

Applications and Effects of Nuclear Reactions Geiger counters, scintillation counters, and film badges are devices used to detect and measure radiation.

Applications and Effects of Nuclear Reactions

Applications of Nuclear Reactions Geiger counters use ionizing radiation, which produces an electric current in the counter, to rate the strength of the radiation on a scale.

Applications of Nuclear Reactions Film badges are often used to monitor the approximate radiation exposure of people working with radioactive materials.

Applications of Nuclear Reactions Scintillation counters measure ionizing radiation.

Applications of Nuclear Reactions With proper safety procedures, radiation can be useful in industry, in scientific experiments, and in medical procedures.

Applications of Nuclear Reactions Nuclear power plants use the process of nuclear fission to produce heat in nuclear reactors. The heat is used to generate steam, which is then used to drive turbines that produce electricity.

Nuclear Reactors

Applications of Nuclear Reactions A radiotracer is a radioisotope that emits non-ionizing radiation and is used to signal the presence of an element or of a specific substance. Radiotracers are used to detect diseases and to analyze complex chemical reactions.

Applications of Nuclear Reactions

Applications of Nuclear Reactions Ionizing radiation has many uses. An X-ray is ionizing radiation, and ionizing radiation can be used in medicine to kill cancerous cells.

Applications of Nuclear Reactions Most medical devices require sterilization after they are packaged, and another trend has been the move to sterilization by gamma radiation as opposed to other methods such as ethylene oxide gas. Advantages of gamma irradiation include speed, cost- effectiveness, and the elimination of the need for special packaging.

Applications of Nuclear Reactions Chemical reaction rates are greatly affected by changes in temperature, pressure, and concentration, and by the presence of a catalyst. In contrast, nuclear reaction rates remain constant regardless of such changes. In fact, the half-life of any particular radioisotope is constant.

Applications of Nuclear Reactions Because of this, radioisotopes, especially carbon-14, can be used to determine the age of an object. The process of determining the age of an object by measuring the amount of a certain radioisotope remaining in that object is called radiochemical dating.

Radiochemical Dating

Effects of Nuclear Reactions Any exposure to radiation can damage living cells. Gamma rays are very dangerous because they penetrate tissues and produce unstable and reactive molecules, which can then disrupt the normal functioning of cells.

Effects of Nuclear Reactions The amount of radiation the body absorbs (a dose) is measured in units called rads and rems. Everyone is exposed to radiation, on average 100–300 millirems per year. A dose exceeding 500 rem can be fatal.