The length of time required for half of the radioactive atoms in a sample to decay is its

HALF LIFE.

Explanation:

The half life of a radioactive element refers to the length of time that is required for the half of the atoms of a particular sample of radioactive element to decay. For instance a radioactive element that a mass of 120 g and a half life of 2 years will be reduce to 60 g in two years. Half life is usually used to measure the rate of disintegration of an unstable isotope. The principle of half life of radioactive elements can be used to determine the relative ages of rocks.  

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i would say that hybridization occurs frequently in nature, because there are many types of hybrids that have occurred naturally, such as the mule (male donkey + female horse)