How do scientist know the half-life of radioactive elements?

Question

eg. Uranium 235 has a half-life of 730 million years,

I am curious how they have learned that. it seems to me it would take a long time to get a meaningful measurement.

Answer 1

First you take a known amount of the radioactive substance whose half-life you want to measure. Then you put it in a counter and count the number of radioactive decays -- that is, atoms that change into other atoms -- per second. From that you can readily calculate the half-life of the substance.

There is nothing very mysterious about it. It's often done in elementary college labs and even sometimes in high-school science labs.

Answer 2

by the carbon dating of it and how much of an element is not nuclear after a certain amount of time

Answer 3

The decay of radioactive elements is constant.  In other words, if one were able to measure how much decay occurred in 10 seconds, one could deduce how much decay will occur in 10 years.  Or in 730 million years.

Answer 4

They measure how much has decayed and put that in a ratio with how much time it took to decay. They then find the half life, or how much time it takes for 1/2 of the sample to decay. For example, if in 1 year, .001 grams of some  1 gram sample of a radioactive element decays, then the chemist would set up a ratio .1 (percent of the sample that decayed) divided by 1 year against 50 (percent of the sample decayed) divided by x (the number of years for half of the sample to decay. They would set this two equations equal to each other, since it should be at the same rate, and solve for x. Using basic algebra, the radioactive element's half life would be (50*1)/.1= 500 yr half life. This can be used with more accurate numbers, to find large numbers like 730 million years.

Answer 5

You measure the initial mass of a sample of the element/isotope, and then measure it as time progresses.  With more measurements (data points), you can graph the results and find the equation that would  produce the particular slope of the line of data points.  Then, you calculate it  out to where half of the sample has decayed down and VOILA you have the half-life.

This is only for substances where the half-life is as long as U235.  Some elements decay in a few seconds or less, and so you encounter other problems in these scenarios.

Answer 6

I think they see an element with half of the parent rock left and see how long its been there for. Idk how they know how long its been there for b/c dating it radioactively would require knowing the length of the half life...../

Answer 7

There are various rules for how many molecules there are in a given volume or a given mass.   Having identified the elements in the material and the path of radioactive decay, it is possible to measure the radiation and determine the half life.  For example, suppose an element Xi breaks down into elements Ya and Za and in the process gives off an Alpha particle.  If I create a pure sample of Xi, or use spectrograph analysis so I know its percentage in a sample and I determine that in the sample there are 1,000,000 atoms.  I measure the radioactivity and find that every second on average there is one alpha particle emitted, then I would calculate that the half life is 500,000 seconds because that is how long it would take half the molecules to break down. (about 5.78 days).    Actually, the calculations are normally done with more complicated mixtures and a variety of reactions going on.

Answer 8

They dont I think its a bunch of c**p. Just like the distance from earth to the sun they dont know that.

Answer 9

With a macroscopic amount of the pure isotope and knowing Avogadro's number and the mass of the sample, the decay rate (counted by Geiger) is proportional to the half-life.  It would only take a long time to get a meaningful measurement if your sample was extremely small, like less than a million atoms.