Chapter 11:Nuclear Chemistry

• Atoms with identical atomic numbers but different mass numbers are called isotopes, and the nucleus of a specific isotope is called a nuclide. ==Thirteen isotopes of carbon are known==.

• Nuclear reactions are different from Chemical reactions in the following ways:

  • A nuclear reaction involves a change in an atom’s nucleus, usually producing a different element.
  • A chemical reaction, by contrast, involves only a change in distribution of the outer-shell electrons around the atom and never changes the nucleus itself or produces a different element.
  • Different isotopes of an element have essentially the same behaviour in chemical reactions but often have completely different behaviour in nuclear reactions.
  • The rate of a nuclear reaction is unaffected by a change in temperature or pressure or by the addition of a catalyst.
  • The nuclear reaction of an atom is essentially the same whether it is in a chemical compound or in an uncombined , elemental form.
  • The energy change accompanying a nuclear reaction can be up to several million times greater than that accompanying a chemical reaction.

• Radioactivity is the spontaneous emission of radiation from a nucleus.

• There were at least two types of radiation, which were named alpha and beta after the first two letters of the Greek alphabet.

• Shortly thereafter, a third type of radiation was found and named for the third Greek letter, gamma.

  • Subsequent studies showed that when the three kinds of radiation are passed between two plates with opposite electrical charges, each is affected differently.

• Alpha radiation bends toward the negative plate and must therefore have a positive charge. Beta radiation, by contrast, bends toward the positive plate and must have a negative charge, whereas gamma radiation does not bend toward either plate and has no charge.

  • Another difference among the three kinds of radiation soon became apparent when it was discovered that a and b radiations are composed of small particles with a measurable mass, whereas gamma (G) radiation consists of high-energy electromagnetic waves and has no mass.
  • Rutherford was able to show that a beta particle is an electron and that an alpha particle is actually a helium nucleus, He2+.
  • A third difference among the three kinds of radiation is their penetrating power.
  • Because of their relatively large mass, alpha particles move slowly (up to about one-tenth the speed of light) and can be stopped by a few sheets of paper or by the top layer of skin.
  • Beta particles, because they are much lighter, move at up to nine-tenth the speed of light and have about 100 times the penetrating power of a particles.
  • Gamma rays move at the speed of light and have about 1000 times the penetrating power of a particles.

• Radioisotope is a radioactive isotope and radionuclide is the nucleus of a radioactive isotope.

• The spontaneous emission of a particle from an unstable nucleus is called nuclear decay, or radioactive decay, and the resulting change of one element into another is called transmutation. ==Nuclear decay: Radioactive element → New element + Emitted partic==le

• Positron is a positive electron, which has the same mass as an electron but a positive charge.

• Electron capture, is a process in which the nucleus captures an inner-shell electron from the surrounding electron cloud, thereby converting a proton into a neutron, and energy is released in the form of gamma rays.

  • The mass number of the product nucleus is unchanged, but the atomic number decreases by one.

• Unstable isotopes that have more protons than neutrons are more likely to undergo beta decay to convert a proton to a neutron, whereas unstable isotopes having more neutrons than protons are more likely to undergo either positron emission or electron capture to convert a neutron to a proton.

  • Also, the very heavy isotopes will most likely undergo alpha decay to lose both neutrons and protons to decrease the atomic number.

• Half-life is the amount of time required for one-half of a radioactive sample to decay.

• Decay series is a sequential series of nuclear disintegrations leading from a heavy radioisotope to a nonradioactive product.

• X rays are like gamma rays; they have no mass and consist of high-energy electromagnetic radiation. The only difference between them is that the energy of X rays is somewhat less than that of gamma rays.

• Cosmic rays are not rays at all but are a mixture of high-energy particles that shower Earth from outer space. They consist primarily of protons, along with some alpha and beta particles.

• The simplest device for detecting exposure to radiation is the photographic film badge worn by people who routinely work with radioactive materials. The film is protected from exposure to light, but any other radiation striking the badge causes the film to fog. At regular intervals, the film is developed and compared with a standard to indicate the radiation exposure.

  • The most versatile method for measuring radiation in the laboratory is the ^^scintillation counter^^, a device in which a substance called a phosphor emits a flash of light when struck by radiation.
  • The number of flashes are counted electronically and converted into an electrical signal.
  • The best-known method for detecting and measuring radiation is the ^^Geiger counter,^^ an argon-filled tube containing two electrodes.
  • The inner walls of the tube are coated with an electrically conducting material and given a negative charge, and a wire in the centre of the tube is given a positive charge.
  • As radiation enters the tube through a thin window, it strikes and ionizes argon atoms, which briefly conduct a tiny electric current between the walls and the centre electrode.
  • The passage of the current is detected, amplified, and used to produce a clicking sound or to register on a meter. The more radiation that enters the tube, the more frequent the clicks.

• Artificial transmutation is the change of one atom into another brought about by a nuclear bombardment reaction.

• Nuclear fission is when heavy nuclei fragment into lighter nuclei and Nuclear fusion is when lighter nuclei combine to form a heavier nuclide.

• The minimum amount of radioactive material needed to sustain a nuclear chain reaction is called critical mass.

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