Atoms may stick together in well-defined molecules or they could be packed together in large arrays.

For students, understanding the general architecture of the atom and the roles played by the main constituents of the atom in determining the properties of materials now becomes relevant.

It is also the time it takes for the count-rate of a substance to reduce to half of the original value.

We cannot predict exactly which atom will decay at a certain time but we can estimate, using the half-life, how many will decay over a period of time.

The half-lives of radioactive isotopes vary between a tiny fraction of a second, and more than 1015 years.

Radioactive substances will give out radiation all the time, regardless of what happens to them physically or chemically.

For example, if one starts with 100 grams of radium 229, whose half-life is 4 minutes, then after 4 minutes only 50 grams of radium will be left in the sample, after 8 minutes 25 grams will be left, after 12 minutes 12.5 grams will be left, and so on.

In physics, a fixed time required for half the radioactive nuclei in a substance to decay.

This would also work if you plotted the number of parent atoms against time.

The following formula can be used to calculate half-life (t1/2): t1/2 = (t ln 1/2)/(ln mf / mi) t = time that has passed mf = the final or remaining mass of undecayed sample mi = the initial or original mass of undecayed sample (The fraction mf / mi is of course equivalent to the fraction of undecayed sample remaining, in case you are given the fraction remaining rather than specific masses.) Note: You can also use base-10 logarithms instead of natural logarithms.