2.2 Half-life Objective 1:i

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Presentation transcript:

2.2 Half-life Objective 1:i Chemistry

Essential Questions What is a half-life? How do you calculate it?

Half Life The half life of a radioactive material is the amount of time required for ½ of the sample to decay into another element or isotope.

Type 1: Finding final g Gold-198 has a half-life of 2.7 days. How much of a 96 g sample of gold-198 will be left after 8.1 days? The half-life is 2.7 days. The time it has been left to decay is 8.1 days. 2.7 goes into 8.7 three times. This means that 3 half-lives have passed. If you are starting with 96 g and three half-lives have passed, then…

Type 1: Finding final g (cont) (1 h.l.) 96/2 = 48 (2 h.l.) 48/2 = 24 (3 h.l.) 24/2 = 12 That means that 12 grams of the 96 grams are left.

Type 2: Finding Total Time Thallium-208 has a half-life of 3.053 min. How long will it take for 120.0 g to decay to 7.50 g? 120.0/2 = 60.0/2 = 30.0/2 = 15.0 g/2 = 7.5 g. Therefore the sample has halved 4 times, or gone through 4 half-lives. If 4 half-lives have passes and you know that the time for a half-life is 3.053 min 4 x 3.053 =12.21 min

Type 3: Finding Half-life If 100.0 g of carbon-14 decays until only 25.0 g of carbon is left after 11,460 y, what is the half-life of carbon-14? 100.0/2 = 50.0/2 = 25.0 Two half-lives have passed. If a total of 11,460 years is TWO half-lives, then 11,460/2 is the half-life: 5,730 years. Answer: The half-life of Carbon-14 is 5,730 years

Half Life (cont) What would the half life be of cesium-137? How much remains after 5 half-lifes? At what time is there 20 g left?

Essential Questions What is a half-life? How do you calculate it?

2.2 Tracked Assignment Worksheet