Question

21.) what does it mean to decay when referring to a half - life of an element?

Explanation:

The half - life of an element in decay refers to the time it takes for half of the initial quantity of a radioactive substance to decay. It is a characteristic property of each radioactive isotope and is used to measure the rate of radioactive decay.

Answer:

The time it takes for half of the initial quantity of a radioactive element to decay.