Chemical isotopes are used in many medical and scientific fields, and radioactive isotopes (radioisotopes)can cause severe damage to plants and animals. Some radioactive isotopes are used in nuclear bombs and nuclear energy production. Most chemical elements can have forms with different atom masses, and each of these atom masses is called an isotope of that element. Atoms consist of protons, neutrons and electrons. The number of protons determines the name of the chemical element, while the number of neutrons (and protons) determines the particular isotope of the element. For example, carbon-12 (12C), carbon-13 (13C) and carbon-14 (14C) are three isotopes of carbon, each containing six protons and also containing six, seven or eight neutrons, respectively. While carbon-12 is the most common form, 13C is magnetically active, and it is therefore useful for magnetic resonance imaging (MRI) and nuclear magnetic resonance spectroscopy. Carbon-14 is radioactive, and is, therefore, useful for detecting very small amounts of material. A typical use of 14C might involve determining all metabolitic endpoints of a new drug candidate. Radioactive isotopes are unstable, and 50% of radioisotopes are converted to another substance in one half-life.