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After completing this lesson, you should be able to:
  • Define selected terms relating to radiation biology.
  • Identify the components of the atom.
  • Describe the effect of x-ray photons upon atoms and molecules.
  • List the harmful effects of overexposure to radiation.



A very basic understanding of radiation biology is necessary for the dental x-ray specialist. In the next few paragraphs, a review of some basic concepts will be discussed along with diagrams representing atomic and molecular configurations. This lesson discusses the interaction and factors influencing cells, tissue, and matter when exposed to ionizing radiation.


  1. Ionization. The gain or loss of electrons from an electrically neutral atomic or molecular configuration caused by radiation. There are other ways whereby electrons may be gained or lost, but we are most concerned with changes brought about by radiation.
  2. Element. A simple substance that cannot be broken down by chemical means. An example of an element is oxygen.
  3. Atom. The smallest unit of an element that still retains the properties of that element.
  4. Compound. A complex substance formed by a chemical union of two or more elements. An example of a compound is water or H2O.
  5. Molecule. The smallest unit of a compound.


The atom is comprised of protons, neutrons, and electrons. The nucleus of an atom contains protons, which have a positive charge. The nucleus also contains neutrons, which are neutral. Electrons, which are negatively charged, revolve or orbit about the nucleus. Generally, protons and electrons in an atom are equal in number. Figure 2-1 represents an atom with one proton, one neutron, and one orbiting electron.

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Figure 2-1. Components of the atom.


The electrons revolve or orbit around the nucleus. They are arranged in shells or orbits much like the planets revolve around the sun (see figure 2-2). The first shell (K shell) can hold one or two electrons but no more. The second shell, or L shell, may contain up to 8 electrons and the third, or M shell, may contain as many as 18 electrons. Some atoms have numerous electrons. To determine the maximum number of electrons in any given shell, the calculations in Table 2-1 can be used. However, no outer shell actually contains more than eight electrons (see figure 2-3).

NOTE: The electrons are shown orbiting the nucleus like planets orbiting a sun for ease of explanation. The actual workings of the atom are a bit more complicated.

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Figure 2-2. Diagram of K, L, and M shells.


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Table 2-1. Determining the maximum number of electrons in any given shell.


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Figure 2-3. Orbiting electrons: 10 electrons.


  1. Atoms. Some atoms are inert or neutral atoms. They have outer shells that are completely filled with electrons. It is extremely difficult for them to combine with other atoms to form molecules. Other atoms are referred to as bonding atoms. They have less than eight electrons in their outer shell (less than two in the K shell) and combine readily with atoms of other bonding atoms. See figure 2-4.

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Figure 2-4. Inert atoms and bonding atoms.

  1. Molecules. Molecules are formed by the combination of two or more atoms. A very common molecular formation is water, resulting from the combination of bonding atoms. Both have outer shells containing less than eight electrons. Two hydrogen atoms combine (or bond) with one oxygen atom to form water or H20. See figure 2-5.

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Figure 2-5. Bonding of one oxygen and two hydrogen atoms (water).


A brief definition of an x-ray photon was previously stated. However, a more comprehensive definition is necessary for the next series of photon actions. X-ray photons are electromagnetic rays produced in the x-ray tube head when electrons from the cathode filament strike the anode target. They are bundles of pure energy. The photons transfer their energy to the substance through which they pass whether it be air, an x-ray film, or the living tissue of the patient or the specialist. They cannot be seen or felt.

  1. Photon Action Upon Atoms.
(1) Photon collision with the nucleus of an atom. The photon may strike the nucleus of an atom. If this occurs, the atom will be destroyed and the photon will release or expend its energy. See figure 2-6.

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Figure 2-6. X-ray photon strikes the nucleus of an atom.

(2) A direct photon hit upon an electron by a photon. The photon may strike an electron with a direct hit. This action will result in the release of the photon's energy, transferring its energy to the electron. The electron will be dislodged from its shell. When an electron is dislodged in this manner, it is called a photo-electron. The dislodged or departing electron (now a photo-electron) will have energy to ionize or strike other electrons (figure 2-7). This is a form of scattered/secondary radiation, as noted earlier in the text.

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Figure 2-7. Direct hit upon an electron by a photon.

(3) An indirect photon hit upon an electron. The photon may strike one of the orbiting electrons with a glancing blow, dislodging the electron from its shell. By striking a glancing blow, the photon will still possess energy and go on to strike other electrons. The dislodged electron becomes a photon-electron and will have energy itself. It, too, may strike and dislodge other electrons (figure 2-8). This is also a form of scattered/secondary radiation.

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Figure 2-8. Indirect photon hit upon an electron.

  1. Photon Action Upon Living Cells.

(1) Living cells are composed of atoms and molecules. If the structure of the atoms and molecules is changed, this may adversely affect the cell. When cells are exposed to ionizing radiation, the structure of some of the atoms and molecules within the cell are changed.

(2) These are some of the effects that ionizing radiation has upon the cell.

(a) Cell death. The x-ray photon may strike a molecule in a sensitive area of a living cell and cause cell death.

(b) Toxic substances. The body is composed of a high percentage of water (H2O). The ionization (gain or loss of positive and/or negative charge) of atoms and molecules results in the breaking of the hydrogen-oxygen bond. When this occurs, there is a reforming of hydrogen and oxygen elements and hydrogen-oxygen compounds. One compound resulting from this restructuring of the atoms and molecules is hydrogen peroxide (H2O2). Hydrogen peroxide is highly toxic to cells. If large amounts of toxic substances (hydrogen-oxygen compounds) are formed, cell death will result.

(c) Mutated cell formation. The chromosomes, which are the blueprint for the formation of new cells, are changed by excessive radiation exposure, resulting in mutated cells. The new mutated cells do not function properly. When a cell is changed in this manner, the life cycle or span of the cell is changed.


  1. Somatic Effects.

(1) Erythema. This is the reddening of the skin much like that of a sunburn; however, radiation exposure affects deeper tissue.

(2) Radiodermatitis. This refers to dry, flaky skin that doesn't heal easily. Ulcerations may become malignant.

(3) Cataracts. An overexposure to the eye could result in cataracts (a clouding of the lens or of its surrounding transparent membrane); however, this effect will appear long after the original exposure.

(4) Cancer. The cause of most natural occurring cancers is unknown. With increased exposure to radiation there is an increase in the incidence of cancer.

(5) Alopecia (epilation). This is hair loss.

  1. Genetic Effects.

(1) Female. The ovaries are especially sensitive to radiation to the female fetus before birth and through childhood. The ovaries decline in sensitivity when the female reaches 20 to 30 years of age. After this time, there is increased sensitivity with increasing age.

(2) Male. Many investigators have recorded normal births from fathers whose testicles had received a radiation dose between 50 and 300 rad. Nevertheless, procreation at any time following such irradiation is ill advised.

(3) An unborn child. Radiation exposure is dangerous to any unborn child. However, the period of greatest danger is between 18 and 45 days of gestation. The results of excessive exposure could result in reduced growth, skeletal malformation, vision problems, and reduced head size which is associated with mental retardation.

David L. Heiserman, Editor

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Revised: June 06, 2015