Different forms

of an element,

each having a

different atomic

entire mass of a sample of radioactive material does not disintegrate

mass.

simultaneously. Relatively few of the great number of nuclei present in an

ordinary sample will disintegrate in any small-time interval. The term 'half

life' is used to indicate the time required for half of the number of radioactive

atoms initially present to disintegrate. When the half life of uranium 234 is

said to be 269,000 years, this means that, on the average, half of the atoms

of uranium present in a given sample will disintegrate in 269,000 years and

half of the remaining atoms will disintegrate by half in the next 269,000

years. The half life of a radioactive element varies form element to element,

ranging from 14 billion years for thorium 232 to a fraction of a second for

polonium 212. Stating the half life is a way of indicating the rate at which the

nuclei of a radioactive element will disintegrate.

The half life of a radioactive element is the time required for

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