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Chapter 42: Q 6 Conceptual Question (page 1235)
Nucleus A decays into nucleus B with a half-life of 10 s. At
t = 0 s, there are 1000 A nuclei and no B nuclei. At what time
will there be 750 B nuclei?
Therefore, at timethere will be 750 B nuclei.
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1.5 Gy of gamma radiation are directed into a 150 g tumour
during radiation therapy. How much energy does the tumour
absorb?
You learned in Chapter 41 that the binding energy of the electron in a hydrogen atom is .
a. By how much does the mass decrease when a hydrogen atom is formed from a proton and an electron? Give your answer both in atomic mass units and as a percentage of the mass of the hydrogen atom.
b. By how much does the mass decrease when a helium nucleus is formed from two protons and two neutrons? Give your answer both in atomic mass units and as a percentage of the mass of the helium nucleus.
c. Compare your answers to parts a and b. Why do you hear it said that mass is “lost” in nuclear reactions but not in chemical reactions?
Stars are powered by nuclear reactions that fuse hydrogen into helium. The fate of many stars, once most of the hydrogen is used up, is to collapse, under gravitational pull, into a neutron star. The force of gravity becomes so large that protons and electrons are fused into neutrons in the reaction . The entire star is then a tightly packed ball of neutrons with the density of nuclear matter.
a. Suppose the sun collapses into a neutron star. What will its radius be? Give your answer in .
b. The sun's rotation period is now 27 days. What will its rotation period be after it collapses?
Rapidly rotating neutron stars emit pulses of radio waves at the rotation frequency and are known as pulsars.
Use the graph of binding energy to estimate the total energy released if three 4 He nuclei fuse together to form a 12 C nucleus
The technique known as potassium-argon dating is used to date old lava flows. The potassium isotope 40 K has a 1.28-billionyear half-life and is naturally present at very low levels. 40 K decays by two routes: 89% undergo beta-minus decay into 40 Ca while 11% undergo electron capture to become 40 Ar. Argon is a gas, and there is no argon in flowing lava because the gas escapes. Once the lava solidifies, any argon produced in the decay of 40 K is trapped inside and cannot escape. A geologist brings you a piece of solidified lava in which you find the 40 Ar/ 40 K ratio to be 0.013. What is the age of the rock?
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