An ion is an atom or molecule which has an unequal positive and negative charge. The negative electrical charge is derived from electrons and the positive charge from protons in the nucleus. An example of an ion is sodium chloride (NaCl). When dissolved in water, sodium and chloride separate to form two kinds of ions: Na⁺ and Cl^–. Sodium loses one electron and chloride gains one electron. Why does this happen? Because the sodium atom has a single electron in the outermost electron orbit, losing it makes the atom more stable. Likewise, the chloride atom has 7 electrons in the outermost electron orbit. Gaining one electron fills the orbit with 8 electrons, which makes the atom more stable. Thus, the give and take of selected electrons is involved in the formation of ions ordinarily.

After radiation exposure, radiation energy is released into absorbing materials, which causes excitation (which propells an electron to a higher level of orbit in an atom) or ionization (which allows an electron to move away from an atom). In contrast to ordinary ion formation mentioned above, radiation can cause the loss of any electron from an atom resulting in highly unstable ions called radicals. Most radicals react with neighboring atoms or molecules almost instantaneously (in less than 1/1,000 of a second).

The human body consists of 70% water. Radiation can break H₂O to produce OH radicals that are known to be the most potent in attacking various biomolecules including DNA, the genetic blueprint. Radiation also can directly hit DNA to produce radicals resulting in DNA strand breaks.

Different types of radiation, such as gamma rays or neutrons, cause different spatial distribution of ionization. Gamma irradiation produces ionizations homogeneously within a cell, whereas neutron irradiation tends to produce clusters of ionization that are believed to cause more serious damage to the cells.