3-6 Nuclear Changes

What Is Natural Radioactivity? Nuclei Losing Particles or Radiation

In addition to physical and chemical changes, matter can undergo a third type of change known as a nuclear change. There are three types of nuclear change: radioactivity, nuclear fission, and nuclear fusion.

Radioactive decay is a nuclear change in which unstable isotopes spontaneously emit fast-moving chunks of matter (called particles), high-energy electromagnetic radiation, or both at a fixed rate. Unstable isotopes are called radioactive isotopes or radioisotopes, and only decays yielding an isotope that is more stable than the original one occur spontaneously.

The radiation emitted by radioisotopes is dam- aging ionizing radiation. The most common form is high-energy electromagnetic radiation known as gamma rays. This radiation is highly penetrating, which makes it very useful in cancer therapy, but it is otherwise best avoided.

High-speed ionizing particles emitted from the nuclei of radioactive isotopes are most commonly of two types:

  1. Alpha particles (fast-moving, positively charged chunks of matter that consist of two protons and two neutrons)

  2. Beta particles (high-speed electrons)

The decay of a given radioisotope into another specified isotope occurs at a fixed and unique characteristic rate. This rate of decay can be expressed in terms of the radioisotope’s half-life: the time needed for one-half of the nuclei in a given quantity of the radioisotope to decay and emit radiation.

Exposure to ionizing radiation from alpha particles, beta particles, and gamma rays can damage cells in two ways:

  1. Genetic damage from mutations or changes in DNA molecules that alter genes and chromosomes. If the mutation is harmful, it can lead to genetic defects in the next generation of offspring or several generations later.

  2. Somatic damage to tissues, which causes harm during the victim’s lifetime.

What Is Nuclear Fission? Splitting Heavy Nuclei

Nuclear fission is a nuclear change in which nuclei of certain isotopes with large mass numbers are split apart into lighter nuclei when struck by neutrons; each fission releases two or three more neutrons and energy.

For these multiple fissions to take place, enough fissionable nuclei must be present to provide the critical mass needed for efficient capture of these neutrons. Multiple fissions within a critical mass form a chain reaction, which releases an enormous amount of energy.

What Is Nuclear Fusion? Forcing Light Nuclei to Combine

Nuclear fusion is a nuclear change in which two isotopes of light elements, such as hydrogen, are forced together at extremely high temperatures until they fuse to form a heavier nucleus. Lots of energy is released when this happens.

Nuclear fusion is much more difficult to initiate than nuclear fission, but once started it releases far more energy per unit of fuel than does fission.