Half Life
a new sample of DNA labeled with ? has an activity of 12500 dpm. an older sample that started with 12500 dpm now has an activity of 3125 dpm. how old is the older sample
2 half lives
what is the approximate half life of the radioactivity shown in this graph?
8 days
Summary So Far
Atom Structure -protons and neutrons in the nucleus -electrons orbit the nucleus the number of protons determines the identity of the atom-which element it is Atoms with the same number of protons but differing numbers of neutrons are isotopes of each other some combinations of protons and neutrons are unstable -> these nuclei are radioactive -> they release particles or energy to become more stable release of energy = heat -> power a generator
Half Life
learning goal #4a-understand what half-life is the decay of an individual unstable nucleus is a random event- when it will happen cant be predicted. however, the behavior of a population of nuclei is very predictable. radioactive decay does not occur at a linear rate - it is described by an exponential curve
Reading graphs
learning goal 4b- determine the half life of an isotope from a graph of radioactivity versus time how can i find the half life of the radioactivity in the sample whose activity is shown in this graph
How long?
learning goal 4c- predict the amount of radioactivity remaining in a sample after a specified amount of time you obtain a pure sample of tritium (hydrogen-3) which decays by beta emission with a half life of 12.3 years after 36.9 years, what fraction of the sample is still tritium? to answer we need to find how many half lives have passed
Time
the half life of a radioisotope is a constant and isnt affected by physical changes- eg, heating or melting- or by chemical reactions the half life varies widely between radioisotopes radioisotope/half life Kr-93/1.3 seconds U-239/23.5 minutes Tc-99m/6 hours Np-239/2.4 days I-131/8 days C-14/5730 years Pu-239/2.4 x 10^4 years U-235/7 x 10^8 years U-238/4.5 x 10^9 years
Exponential Decay
what is constant is the time it takes for half of the nuclei of a particular isotope to decay half life- how long it takes for half of the nuclei to decay the half life is different (and characteristic) for each isotope after 10 half lives, the amount of radioactivity left is 1/1024 (0.1%) of the original