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From Wikipedia

Isotopes of chlorine

Chlorine(Cl) has isotopes with mass numbers ranging from 32 g mol−1 to 40 g mol−1. There are two principal stable isotopes, 35Cl (75.76%) and 37Cl (24.24%), found in the relative proportions of 3:1 respectively, giving chlorine atoms in bulk an apparent atomic weight of 35.5.
Standard atomic mass: 35.453(2) u

Chlorine-36 (36Cl)

NO amounts of radioactive36.0Cl exist in the environment, in a ratio of about 7x10−13 to 1 with stable isotopes. 36Cl is produced in the atmosphere by spallation of 36Ar by interactions with cosmic rayprotons. In the subsurface environment, 36Cl is generated primarily as a result of neutron capture by 35Cl or muon capture by 40Ca. 36Cl decays to 36S and to 36Ar, with a combined half-life of 308,000 years. The half-life of this hydrophilic nonreactive isotope makes it suitable for geologic dating in the range of 60,000 to 1 million years. Additionally, large amounts of 36Cl were produced by irradiation of seawater during atmospheric detonations of nuclear weapons between 1952 and 1958. The residence time of 36Cl in the atmosphere is about 1 week. Thus, as an event marker of 1950s water in soil and ground water, 36Cl is also useful for dating waters less than 50 years before the present. 36Cl has seen use in other areas of the geological sciences, forecasts, and elements.

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From Yahoo Answers

Question:The half-life of a radioactive isotope is 350 years. If there are currently 4 kilograms of the isotope, how many kilograms will be left after 1000 years? Use the formula A(t) = Ca^kt and round your answer to the nearest tenth.

Answers:i think you mean A(t) = Ce^kt A(t)/C = e^kt 0.5 = e^350k 350k = ln 0.5 k = ln 0.5/350 = - 0.00198 A(1000) = 4e^(-0.00198*1000) = 0.552 => 0.6 g ---------------------- p.s: ------ if you didn't specify the formula, one line ans is A(1000) = 4*0.5^(1000/350) = 0.552 => 0.6 g

Question:How does knowing the half-life of a radioactive isotope tell us the age of a rock? Im trying to help my little brother out with his homework but it would help if i knew the answer first so could someone give me a little help?

Answers:If you have a rock sample with a radioactive isotope present, it's fairly simple to work out the age of the sample. First off, I'll define half-life. The half-life of an isotope is the amount of time that it takes for one half of a sample to decay. Find out what the daughter isotope or element is. This is what the radioactive isotope will become after it decays. For example, Carbon-14 decays into Carbon-12. Next, find the ratio of parent isotope to daughter and parent isotope. So, if there were 2 grams of C14 and 2 grams of C12, the ratio would be 2 to 4 (2:4) or 1 to 2 (1:2). In fraction form, this would be one half, so one half of the C14 has decayed. this means one half life has gone by. If the half life is about 5000 years, then the sample is 5000 years old.

Question:(This isotope and its breakdown product do not normally occur together when molten rock cools and becomes solid. If you have a volcanic rock sample containing 0.25 grams of the radioactive isotope and 0.75 grams of its breakdown product) a) 2,500 years ago b) 50,000 years ago c) 10,000 years ago d) 20,000 years ago

Answers:d) 20,000 years ago The amount of radioactive isotope decreases by 1/2 every half life (10,000 years). The amount that is "lost" becomes the breakdown product. When formed (0 years elapsed) 1.00 grams of radioactive isotope 0.00 grams of breakdown product After 1 half life (10,000 years elapsed) 0.50 grams of radioactive isotope 0.50 grams of breakdown product After 2 half lives (20,000 years elapsed) 0.25 grams of radioactive isotope 0.75 grams of breakdown product

Question:The half-life of a particular radioactive isotope is 140 days. How many days would it take for the decay rate of a sample of this isotope to fall to one quarter of its initial value?

Answers:Hello Smily Liu, I can't understand why you have inserted the word 'decay rate'. If suppose I understand that the ratio of the remiainig to the initial is one quarter, then the days needed to attain that stage will be two half lives. Because 1/4 remains means (1/2)^2. Hence 2 half lives. So the duaration will be 2*140 = 280 days.

From Youtube

Rate of radioactive decay: A worked example to calculate the half life of an isotope :This worked example shows step by step, how to calculate the half life of an isotope. Calculating the half-life of a radioactive isotope has many applications not just in chemistry but in physics, environmental science and medicine. The worked example shows how easy it is to use the intergrated first order rate law in order to find the half life of an isotope...

Half-Life of a Radioactive Element :demonstrations.wolfram.com The Wolfram Demonstrations Project contains thousands of free interactive visualizations, with new entries added daily. The half-life of a radioactive element is the time needed for half of the material to decay. The blue and orange points represent the original number of radioactive nuclei and those that decay; the number of blue points decreases by half at each step in... Contributed by: Enrique Zeleny