CHEM 2A Class Pack F17 9. If a nucleus undergoes positron emission, how will the

Question

CHEM 2A Class Pack F17 9. If a nucleus undergoes positron emission, how will the atomic number change? How will the mass number change? 10. If a radioactive sample has a halif-life of 2.5 years, how much of a 10.0g sample is left after 10 years? After 3 half-lives? 11. A sample of Ur-235, a radioactive isotope, is monitored as a function of time. Radioactive elements have "half-lives". A half-life is the amount of time it takes for half of the sample to decay The Decay Uraniu 235 with Time 1. How many atoms were originally in the sample? Approximately how long does it take for half of the sample to decay? This is the half- life 2. 3. How much time does it take for only 500, 000 atoms to remain? 4. How many atoms will remain in 2 billion years

Explanation / Answer

9. For a positron emission, the atomic number increases by one. There is no increase in the mass number.

10. t1/2 = 2.5 years

Initial Sample = 10.0 g

The initial sample is reduced by :1/(2)n where n is the number of half life

n = 4 half lives

Amount left after 4 half lives = 10.0 g x 1/(2)4 = 10.0 / 16 = 0.625 g

After three half lives

Amount left after 3 half lives = 10.0 g x 1/(2)3 = 10.0 / 8 = 1.25 g

11.

1. The number of atoms in the orginal sample = 2,000,000.

2. 0.75 billion years is the time taken for half of the sample to decay.

3. Two half lives or 1.5 billion years is the time taken for 500, 000 atoms to remain.

4. The number of atoms that will remain in 2 billion years: 200,000 atoms (approximatley from graph)

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