|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
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.