radioactive decay

types of radioactive decay

There are three types of radioactive decay: alpha, beta, and gamma decay. In all three types, nuclei emit radiation, but the nature of that radiation differs from one type of decay to another. You can watch a video about the three types at this URL: (17:02). MEDIA Click image to the left or use the URL below. URL:

comparing types of radiation

The different types of radiation vary in how far they are able to travel and what they can penetrate (see Figure 11.8 and the URL below). MEDIA Click image to the left or use the URL below. URL: Alpha particles can travel only a few centimeters through air. They cannot pass through a sheet of paper or thin layer of clothing. They may burn the skin but cannot penetrate tissues beneath the skin. Beta particles can travel up to a meter through air. They can pass through paper and cloth but not through a sheet of aluminum. They can penetrate and damage tissues beneath the skin. Gamma rays can travel thousands of meters through air. They can pass through a sheet of aluminum as well as paper and cloth. They can be stopped only by several centimeters of lead or several meters of concrete. They can penetrate and damage organs deep inside the body.

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gamma decay

In alpha and beta decay, both particles and energy are emitted. In gamma decay, only energy is emitted. Gamma decay occurs when an unstable nucleus gives off gamma rays. Gamma rays, like rays of visible light and X-rays, are waves of energy that travel through space at the speed of light. Gamma rays have the greatest amount of energy of all such waves. By itself, gamma decay doesnt cause one element to change into another, but it is released in nuclear reactions that do. Some of the energy released in alpha and beta decay is in the form of gamma rays. You can learn more about gamma radiation at this URL: (2:45). MEDIA Click image to the left or use the URL below. URL:

alpha decay

Alpha decay occurs when an unstable nucleus emits an alpha particle and energy. The diagram in Figure 11.6 represents alpha decay. An alpha particle contains two protons and two neutrons, giving it a charge of +2. A helium nucleus has two protons and two neutrons, so an alpha particle is represented in nuclear equations by the symbol 4 He. 2 The superscript 4 is the mass number (2 protons + 2 neutrons). The subscript 2 is the charge of the particle as well as the number of protons. An example of alpha decay is the decay of uranium-238 to thorium-234. In this reaction, uranium loses two protons and two neutrons to become the element thorium. The reaction can be represented by this equation: 238 92 U 4 !234 90 Th +2 He + Energy If you count the number of protons and neutrons on each side of this equation, youll see that the numbers are the same on both sides of the arrow. This means that the equation is balanced. The thorium-234 produced in this reaction is unstable, so it will undergo radioactive decay as well. The alpha particle (42 He) produced in the reaction can pick up two electrons to form the element helium. This is how most of Earths helium formed. Problem Solving ? 4 Problem: Fill in the missing subscript and superscript to balance this nuclear equation: 208 84 Po !? Pb +2 He + Energy Solution: The subscript is 82, and the superscript is 204. You Try It! ? 4 Problem: Fill in the missing subscript and superscript to balance this nuclear equation: 222 ? Ra !86 Rn+2 He+Energy

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beta decay

Beta decay occurs when an unstable nucleus emits a beta particle and energy. A beta particle is an electron. It has a charge of -1. In nuclear equations, a beta particle is represented by the symbol 01 e. The subscript -1 represents the particles charge, and the superscript 0 shows that the particle has virtually no mass. Nuclei contain only protons and neutrons, so how can a nucleus emit an electron? A neutron first breaks down into a proton and an electron (see Figure 11.7). Then the electron is emitted from the nucleus, while the proton stays inside the nucleus. The proton increases the atomic number by one, thus changing one element into another. An example of beta decay is the decay of thorium-234 to protactinium-234. In this reaction, thorium loses a neutron and gains a proton to become protactinium. The reaction can be represented by this equation: 234 90 Th !234 91 Pa + 0 1 e + Energy The protactinium-234 produced in this reaction is radioactive and decays to another element. The electron produced in the reaction (plus another electron) can combine with an alpha particle to form helium. Problem Solving Problem: Fill in the missing subscript and superscript in this nuclear equation: 131 I 53 !?? Xe + 14 C ? !?7 N + Solution: The subscript is 54, and the superscript is 131. 0 e + Energy 1 You Try It! Problem: Fill in the missing subscript and superscript in this nuclear equation: 0 e + Energy 1

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radioactive dating

Radioactive isotopes can be used to estimate the ages of fossils and rocks. The method is called radioactive dating. Carbon-14 dating is an example of radioactive dating. It is illustrated in the video at this URL: MEDIA Click image to the left or use the URL below. URL:

rate of radioactive decay

A radioactive isotope decays at a certain constant rate. The rate is measured in a unit called the half-life. This is the length of time it takes for half of a given amount of the isotope to decay. The concept of half-life is illustrated in Figure 11.9 for the beta decay of phosphorus-32 to sulfur-32. The half-life of this radioisotope is 14 days. After 14 days, half of the original amount of phosphorus-32 has decayed. After another 14 days, half of the remaining amount (or one-quarter of the original amount) has decayed, and so on. Different radioactive isotopes vary greatly in their rate of decay. As you can see from the examples in Table 11.1, the half-life of a radioisotope can be as short as a split second or as long as several billion years. You can simulate radioactive decay of radioisotopes with different half-lives at the URL below. Some radioisotopes decay much more quickly than others. Isotope Uranium-238 Potassium-40 Carbon-14 Hydrogen-3 Radon-222 Polonium-214 Half-life 4.47 billion years 1.28 billion years 5,730 years 12.3 years 3.82 days 0.00016 seconds Problem Solving Problem: If you had a gram of carbon-14, how many years would it take for radioactive decay to reduce it to one-quarter of a gram? Solution: One gram would decay to one-quarter of a gram in 2 half-lives years. 1 2 12 = 1 4 , or 2 5,730 years = 11,460 You Try It! Problem: What fraction of a given amount of hydrogen-3 would be left after 36.9 years of decay?

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limit on carbon14 dating

Carbon-14 has a relatively short half-life (see Table 11.1). After about 50,000 years, too little carbon-14 is left in a fossil to be measured. Therefore, carbon-14 dating can only be used to date fossils that are less than 50,000 years old. Radioisotopes with a longer half-life, such as potassium-40, must be used to date older fossils and rocks.

how carbon14 dating works

Carbon-14 forms naturally in Earths atmosphere when cosmic rays strike atoms of nitrogen-14. Living things take in and use carbon-14, just as they do carbon-12. The carbon-14 in living things gradually decays to nitrogen-14. However, it is constantly replaced because living things keep taking in carbon-14. As a result, there is a fixed ratio of carbon-14 to carbon-12 in organisms as long as they are alive. This is illustrated in the top part of Figure 11.10. After organisms die, the carbon-14 they already contain continues to decay, but it is no longer replaced (see bottom part of Figure 11.10). Therefore, the carbon-14 in a dead organism constantly declines at a fixed rate equal to the half-life of carbon-14. Half of the remaining carbon-14 decays every 5,730 years. If you measure how much carbon- 14 is left in a fossil, you can determine how many half-lives (and how many years) have passed since the organism died.

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instructional diagrams

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questions

An alpha particle has the same mass as a helium nucleus.

-->  a. true

b. false

How does a nucleus change when it undergoes beta decay?

-->  a. Its atomic number increases.

b. Its mass number increases.

c. It has more neutrons.

d. It has fewer protons.

Nuclear equations do not need to balance.

a. true

-->  b. false

Which of the following nuclear equations represents alpha decay?

a. 146 C 147 N +10 e + Energy

b. 210

-->  c. 92 U  90 Th +2 He + Energy

d. two of the above

A beta particle has virtually no mass.

-->  a. true

b. false

For a nuclear equation to be balanced, both sides of the equation must have the same number of

a. protons.

b. neutrons.

c. electrons.

-->  d. protons plus neutrons.

Charged particles are emitted from a nucleus during

a. beta decay.

b. alpha decay.

c. gamma decay.

-->  d. two of the above

Gamma rays are released only during gamma decay.

a. true

-->  b. false

Which of the following radioisotopes has the longest half-life?

-->  a. uranium-238

b. carbon-14

c. hydrogen-3

d. radon-222

Alpha particles can pass through a sheet of aluminum.

a. true

-->  b. false

Carbon-14 has a half-life of 5.7 million years.

a. true

-->  b. false

All three types of radioactive decay emit energy.

-->  a. true

b. false

A beta particle has a charge of +1.

a. true

-->  b. false

During beta decay, a proton is emitted by a nucleus.

a. true

-->  b. false

All radioisotopes decay at the same constant rate.

a. true

-->  b. false

In all three types of radioactive decay, nuclei emit energy.

-->  a. true

b. false

During gamma decay, one element changes into another.

a. true

-->  b. false

Most of Earths helium formed when alpha particles picked up electrons.

-->  a. true

b. false

Carbon-14 forms when cosmic rays strike atoms of carbon-12.

a. true

-->  b. false

Carbon-14 dating can be used to estimate the age of any fossil.

a. true

-->  b. false

method of aging fossils that uses radioisotopes

a. half-life

b. alpha particle

-->  c. radioactive dating

d. beta particle

e. gamma decay

f. radioactive decay

g. gamma ray

process in which unstable nuclei emit charged particles and energy

a. half-life

b. alpha particle

c. radioactive dating

d. beta particle

e. gamma decay

-->  f. radioactive decay

g. gamma ray

process in which a radioactive nucleus emits only energy

a. half-life

b. alpha particle

c. radioactive dating

d. beta particle

-->  e. gamma decay

f. radioactive decay

g. gamma ray

form of energy emitted during radioactive decay

a. half-life

b. alpha particle

c. radioactive dating

d. beta particle

e. gamma decay

f. radioactive decay

-->  g. gamma ray

particle consisting of two protons and two neutrons

a. half-life

-->  b. alpha particle

c. radioactive dating

d. beta particle

e. gamma decay

f. radioactive decay

g. gamma ray

rate at which a radioactive isotope decays

-->  a. half-life

b. alpha particle

c. radioactive dating

d. beta particle

e. gamma decay

f. radioactive decay

g. gamma ray

electron emitted by an unstable nucleus

a. half-life

b. alpha particle

c. radioactive dating

-->  d. beta particle

e. gamma decay

f. radioactive decay

g. gamma ray

Unstable nuclei of radioisotopes may become stable by

a. undergoing radioactive decay.

b. changing into other elements.

c. emitting particles and energy.

-->  d. all of the above

Which of the following equations represents alpha decay?

a. 146 C  147 N + 10 e + Energy

-->  b. 222

c. 53 I  54 Xe + 1 e + Energy

d. none of the above

Examples of beta decay include

a. 238

b. 234

c. 106 Sg  104 Rf + 2 He + Energy

-->  d. two of the above

Which of the following radioisotopes has the shortest half-life?

a. uranium-238

-->  b. potassium-40

c. carbon-14

d. polonium-214

Beta particles can travel

a. only a few centimeters through air.

b. up to a meter through air.

-->  c. through several meters of concrete.

d. for thousands of meters through air.

Which type of radiation is most harmful to living things?

a. alpha particles

b. beta particles

c. gamma rays

-->  d. X rays

An alpha particle has a charge of

-->  a. 0

b. -1

c. +1

d. +2

diagram questions

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