An isotope's half-life is

A) the amount of time that the isotope has been on Earth divided by two.
B) the length of time it takes for half of a sample of atoms to decay.
C) the number of protons and neutrons in an atom divided by two.
D) the ratio of unstable atoms to stable atoms in half a gram of sample.

Half-Life

The time required for half the quantity of a radioactive substance to undergo decay or for a drug's concentration in the body to be reduced by half.

Isotope

Variants of a particular chemical element that differ in neutron number, and therefore in mass, but share the same chemical properties.

Atoms Decay

A natural process by which the nucleus of an unstable atom loses energy by emitting radiation, leading to the transformation of the atom into a different element or isotope.