Chapter 33: The Atomic Nucleus and Radioactivity

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Chapter 33: The Atomic Nucleus and Radioactivity
Review questions pg. 658
5.
How do the electric charges of alpha, beta and gamma rays differ?
Ans. The alpha 'ray' consists of alpha particles. Each alpha particle has a +
2 charge.
The beta 'ray' consists of electrons. Each electron has a -1 charge. A
magnetic field will push the oppositely charged particles in opposite
directions.
The gamma ray consists of photons of light. They are not charged at
all.
6.
How does the source differ for a beam of gamma rays and a beam of X
rays.
Ans. Gamma rays come from the nuclei of some atoms. X rays come from
the reconfiguration of electrons surrounding the nucleus of an atom. They
may also be produced when an electron undergoes a large acceleration.
7.
Give two examples of a nucleon.
Ans. Protons and neutrons are found in the nuclei of atoms and are therefor
called nucleons.
11.
Give the atomic number for deuterium and for tritium.
Ans. Deuterium and tritium are both isotopes of hydrogen. Deuterium has
one proton and one neutron while tritium has one proton and two neutrons.
The both have atomic number 1.
Extra: How does the mass of a nucleon compare with the mass of an electron.
Ans. One nucleon is approximately 1800 times more massive than an
electron.
Extra: When beta emission occurs, what change takes place in an atomic
nucleus?
Ans. Beta emission occurs when a neutron emits an electron. The neutron
changes into a proton in the process. The atomic nucleus now has one more
proton that before the emission and thus is now an atom of a different
element.
13.
Distinguish between an isotope and an ion.
Ans. An isotope of an element has a different number of neutrons than a
different isotope of the same element. An ion is a charged atom. It is charged
because it does not have the same number of protons as electrons.
18.
What is meant by radioactive half-life?
Ans. Radioactive half-life is the time required for one half the remaining
radioactive nuclei to undergo radioactive decay.
24.
When thorium, atomic number 90, decays by emitting an alpha
particle, what is the atomic number of the resulting nucleus. What happens to
its atomic mass?
Ans. An alpha particle consists of two protons and two neutrons. When
thorium undergoes alpha decay, the remaining nucleus will have 88 protons
instead of 90. The new atom will be atomic number 88, which is radium-a
different element than before. The alpha particle consists of two protons and
two neutrons. Alpha decay reduces the atomic mass by four.
Ch. 33 Review questions continued. Pg. 658-659
25.
When thorium decays by emitting a beta particle(an electron), what is
the atomic number of the resulting nucleus? What happens to its atomic
mass?
Ans. When a nucleus undergoes beta decay, one of its neutrons changes into
a proton as it emits an electron. Therefore, the atomic number increases by
one. The new atomic number will be 91. Although the fleeing electron
carries a tiny bit of mass away with it, the atomic mass of the atom does not
change.
26.
How does the atomic mass change for questions 24 and 25?
Ans. I included this answer in the answers to # 24 and #25.
Chapter 33: The Atomic Nucleus and Radioactivity
27.
What is the effect on the makeup of a nucleus when it emits an alpha
particle? A beta particle? A gamma ray?
Ans. When the nucleus of an atom emits an alpha particle, it loses two
protons and two neutrons. When the nucleus of an atom emits a beta particle a
neutron changes to a proton. When the nucleus of an atom emits a gamma ray
the nucleus reconfigures itself to a less energetic state.
35.
Ans.
Which isotope of carbon is radioactive? Carbon-12 or Carbon -14
Carbon-14 is a radioactive isotope of carbon.
37.
Why is there more C-14 in new bones than there is in old bones of the
same mass?
Ans. Carbon-14 changes to Nitrogen-14 with a half-life of 5,730 years. So
the amount of Carbon-14 present in a substance is reduced over time
Chapter 33: The Atomic Nucleus and Radioactivity
Exercises pg. 659
1.
X rays are most similar to which of the following: alpha, beta, or
gamma?
Ans. X rays and gamma rays are most similar because they are both photons
of light. The others are not.
3.
Some people say that all things are possible. Is it at all possible for a
hydrogen nucleus to emit an alpha particle? Explain your answer.
Ans. A hydrogen nucleus contains only one proton and zero, one or two
neutrons. An alpha particle consists of two protons and two neutrons.
Therefore a hydrogen atom cannot emit an alpha particle. It cannot emit what
it doesn't have.
4.
Why are alpha and beta rays deflected in opposite directions in a
magnetic field? Why aren't gamma rays deflected?
Ans. Alpha rays consist of positively charged helium nuclei. Beta rays
consist of negatively charged electrons. Gamma rays are uncharged photons
of light. A magnetic field will apply a force to a moving charged particle.
Positively charged particles are accelerated in one direction and negative
charged particles are accelerated in the opposite direction. Because gamma
rays are not charged, they are unaffected by the magnetic field.
5.
The alpha particle has twice the electric charge of the beta particle but,
for the same velocity, accelerates less than the beta in a magnetic field. Why?
Ans. From Newton's second law of motion we know that acceleration is
directly proportional to the net force applied to an object and inversely
proportional to the objects mass. Although the force applied to the alpha
particle is twice that applied to the beta particle, the alpha particle is
approximately 3600 times more massive than the beta.
6.
How do the paths of alpha, beta and gamma radiation compare in an
electric field?
Ans. The positively charged alpha particle will accelerate in the direction of
the magnetic field, the negatively charged beta will accelerate in the opposite
direction of the field and the gamma photon will not be effected by the electric
field.
Force on positively charged
alpha particle
Electric field direction
Force on negatively charged beta
particle
Chapter 33: The Atomic Nucleus and Radioactivity
Exercises pg. 659
7.
Which type of radiation results in the greatest change in atomic mass?
Atomic number?
Ans. Alpha radiation. Alpha radiation. The resulting nucleus will be
missing two protons and two neutrons. The atomic mass will be four less than
the original and the atomic number will be two fewer than the original.
8.
Which type of radiation results in the least change in atomic mass?
The least change in atomic number?
Ans. Gamma radiation. There is no change in mass number or atomic
number because a gamma ray is a photon of light.
10.
In bombarding atomic nuclei with proton "bullets", why must the
protons be accelerated to high energies if they are to make contact with the
target nuclei?
Ans. Atomic nuclei are positively charged. The proton "bullets" are
positively charged. They will be repelled away from each other by the
electromagnetic force.
20.
The amount of radiation from a point source is inversely proportional
to the distance from the source. If a Geiger counter 1 meter from a small
sample reads 360 counts per minute, what will be its counting rate 2 meters
from the source? 3 meters from the source?
Ans. Doubling the distance will result in a count of (1/2)2 = 1/4 the original
count. 1/4 of 360 = 90 counts per minute. Tripling the distance will result in
(1/3)2 = 1/9 the original count. 1/9 of 360 = 40 counts per minute.
24.
When 226
88 Ra decays by emitting an alpha particle, what is the atomic
number of the resulting nucleus? What is the name of the element?
Ans. When the nucleus of an atom emits an alpha particle, it loses two
protons and two neutrons. The remaining nucleus will be atomic number 86
and its mass number will be 222. The reaction can be written as follows:
226
88
Ra
222
86
4
Ra + 2 He
Chapter 33: The Atomic Nucleus and Radioactivity
Exercises pg. 660 continued
25.
When 218
84 Po emits a beta particle, it transforms into a new element. A) What
are the atomic number and atomic mass of this new element? B) What are atomic
number and atomic mass if the polonium instead emits an alpha particle?
Ans. A) Beta emission occurs when a neutron emits an electron as it changes into a
proton. When 218
84 Po emits a beta particle, its atomic number increases by one and its
atomic mass remains unchanged. The resulting atom will be atomic number 85 and its
atomic mass is 218. The reaction can be written as follows:
218
84
Po
218
85
At +
0
1
 where
0
1
 represents the emitted electron.
B) When the nucleus of an atom emits an alpha particle, it loses two protons and two
neutrons. If 218
84 Po emits an alpha particle its new atomic number will be 82 and its new
atomic mass will be 214. The reaction can be written as follows:
218
84
Po
214
82
4
Pb + 2 He
26.
State the number of protons and neutrons in each of the following nuclei:
2
12
56
197
90
238
H,
C,
1
6
26Fe, 79Au, 38Sr, and 92 U.
Ans. Hydrogen 2 has 1 proton and 1 neutron.
Carbon 12 has 6 protons and 6 neutrons.
Iron 56 has 26 protons and 30 neutrons.
Gold 197 has 79 protons and 118 neutrons.
Strontium 90 has 38 protons and 52 neutrons.
Uranium 238 has 92 protons and 146 neutrons.
27.
How is it possible for an element to decay forward in the periodic table-that is,
to decay to an element of higher atomic number?
Ans. When the nucleus of an atom of an element undergoes beta decay, one of its
neutrons changes to a proton as it emits an electron. This will increase the number of
protons and therefor the atomic number, by one.
Prob.1
If a sample of a radioactive isotope has a half-life of 1 year, how much
of the original sample will be left:
At the end of one year?
Ans. 1/2
At the end of two years?
Ans. 1/4
At the end of three years?
Ans. 1/8
Chapter 33: The Atomic Nucleus and Radioactivity
Prob. 2
A sample of a particular radioisotope is placed near a Geiger counter,
which is observed to register 160 counts per minute. Eight hours later the detector
counts at a rate of 10 counts per minute. What is the half-life of the material?
Ans. The half-life is 2 hours. Here is my reasoning. If you cut 160 in half you will
have 80. 1/2 of 80 = 40. 1/2 of 40 = 20. 1/2 of 20 = 10. We repeated this process 4
times. Four half-lives have elapsed. Eight hours divided by 4, equals 2 hours.

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