Radioactivity decays with time. The time required for a radioactive material to lose half of its activity is its half-life. Some isotopes have less than 0.50 seconds of half-life. Carbon 14 is found in the upper atmosphere which is created by cosmic ray bombardment. Carbon 14 mixes with Carbon12 in atmosphere and becomes part of the plant and animals.
Most isotopes found in nature are stable.
Examples for natural unstable isotopes are radium, thorium and uranium. Natural radioactive materials are found in rocks, soil and wood. Most unstable isotopes are formed artificially which disintegrate anywhere form couple of seconds to hundreds of years. The principal way by which an ionizing radiation dissipates its energy in matter is by ejection of one or more orbital electrons. The removal of orbital electrons from an atom is called ionization.