In the process of ionization, an electrically neutral atom becomes either positively or negatively charged. Some substances are easier to ionize than others. This depends on the ionization energy.  Ionizing radiation is a common cause, but ionization can also be done artificially.
- Encyclopædia Britannica. Ionization (chemistry and physics). Encyclopedia Britannica Online, 13 May 2014. <http://www.britannica.com/EBchecked/topic/293007/ionization>.