subatomic physicssection 1 © houghton mifflin harcourt publishing company preview section 1 the...

Subatomic Physics Section 1

© Houghton Mifflin Harcourt Publishing Company

Preview

Section 1 The Nucleus

Section 2 Nuclear Decay

Section 3 Nuclear Reactions

Section 4 Particle Physics



The student is expected to:TEKS

5A research and describe the historical development

of the concepts of gravitational, electromagnetic,

weak nuclear, and strong nuclear forces

5H describe evidence for and effects of the strong

and weak nuclear forces in nature

8C describe the significance of mass-energy

equivalence and apply it in explanations of

phenomena such as nuclear stability, fission, and

fusion



What do you think?

• What holds a nucleus together?• What particles exist within the nucleus?• What force(s) exist between these particles?• Are these forces attractive or repulsive?



The Nucleus

• The chemical symbol for an element is written like the one shown to the left. What information is provided by this symbol?– The atomic number (Z) or

number of protons is 13.– The mass number (A) or number

of protons + neutrons is 27.– The number of neutrons (N) is

14 (27 – 13).– The element is aluminum.



Isotopes

• Isotopes are atoms of the same element with different atomic masses.– The number of neutrons is different.

• Most carbon nuclei have 6 protons and 6 neutrons and an atomic mass of 12.– Called carbon-12– Others have 5 neutrons (carbon-11), 7 neutrons

(carbon-13), or 8 neutrons (carbon-14).



Click below to watch the Visual Concept.

Visual Concept

Isotopes



Nuclear Mass

• The density of the nucleus is approximately 2.3 1017 kg/m3.

• Mass is measured in unified mass units (u).– 1 u is one-twelfth the mass of one

atom of carbon-12.

• 1 u = 1.6605 10-27 kg• Protons and neutrons each

have a mass of approximately 1 u.



Nuclear Mass

• Find the energy equivalent of 1 u in both J and eV. (For c, use the value 2.9979 108 m/s.)– Answers:

• 1.4924 10-44 J, 931.47 106 eV or 931.47 MeV

• With more significant figures, 1 u = 931.49 MeV.• The mass of subatomic particles is often expressed in

MeV.



Nuclear Mass

• This table provides the mass and rest energy of atomic particles in kilograms, unified mass units, and MeV.



Nuclear Stability

• What type of electric force would exist in the nucleus shown?– Protons would repel other protons

very strongly because the distance between them is small.

– Neutrons would produce no forces.

• What holds the nucleus together?– A force called the strong force:

a powerful attractive force between all particles in the nucleus

• Does not depend on charge

• Exists only over a very short range



Nuclear Stability

• As more protons are added to the nucleus, more repulsion exists.– Larger and larger nuclei require more neutrons, and

more strong force, to maintain stability.

• Look at a periodic table to find out which elements have approximately a 1:1 ratio between neutrons and protons, and which elements have the highest ratio of neutrons to protons.




Visual Concept

Nuclear Stability and Ratio of Neutrons and Protons



Binding Energy

• The nucleons (protons and neutrons) have a greater mass when unbound than they do after binding to form a nucleus.– Called binding energy– This energy is released when the binding occurs, and must be

absorbed to separate the nucleons.



Classroom Practice Problem

• The mass of the individual particles in an atom is the mass of the protons, neutrons, and electrons.– For the mass of the protons and electrons combined,

simply multiply the atomic number times the mass of a hydrogen atom (1 electron bound to 1 proton).

• Find the binding energy (in u and MeV) for a helium atom with two protons and two neutrons. The atomic mass of helium-4 is 4.002602 u.– Answer: 0.030378 u or 28.297 MeV



Now what do you think?

• What holds a nucleus together?• What particles exist within the nucleus?• What forces exist between these particles?• Are these forces attractive or repulsive?• What happens to each of these forces when the

particles are farther and farther apart?

• What is meant by the term binding energy?




8D give examples of applications of atomic and

nuclear phenomena such as radiation therapy,

diagnostic imaging, and nuclear power and

examples of applications of quantum phenomena

such as digital cameras



What do you think?

• Often scientists use radioactive carbon dating to determine the age of fossils. • What does the term radioactive mean?• Are all atoms radioactive?

• If not, how are radioactive atoms different from those that are not radioactive?

• How can radioactivity be used to determine the age of a fossil?



Nuclear Decay

• When nuclei are unstable, particles and photons are emitted.– The process is called radioactivity.– It occurs because the nucleus has too many or

too few neutrons.– Three types of radiation can occur:

• Alpha• Beta• Gamma



Alpha Decay ()

• An alpha particle (2 protons and 2 neutrons) is emitted from the nucleus. particles are helium-4 nuclei.– A new element is formed by alpha decay.

– Example of alpha decay:

• The uranium atom has changed into a thorium atom by ejecting an alpha particle.

238 234 492 90 2U Th + He



Radioactive Decay

• These rules are used to determine the daughter nucleus when a parent nucleus decays.

• Note how these rules apply in the alpha decay of uranium-238:– 238 = 234 + 4– 92 = 90 + 2

238 234 492 90 2U Th + He



Beta Decay

• An electron or positron is emitted from the nucleus.– A positron is the same as an electron but with an

opposite charge.– A positron is the antiparticle of an electron.

• Since there are no electrons or positrons in the nucleus, how can beta decay occur?– A neutron is transformed into a proton and an electron,

and then the electron is ejected.– A proton is transformed into a positron and a neutron,

and then the positron is ejected.



Beta Decay

• It was discovered that, during beta decay, momentum and energy were not conserved.– The ejected electron did not have as much forward momentum as

the recoiling nucleus.

• In 1930, Wolfgang Pauli proposed the existence of a particle that was not detectable at the time.

• In 1956, Pauli’s neutrino () was detected.• The neutrino and its antiparticle, the antrineutrino (), are

emitted during beta decay.– Electrons are accompanied by antineutrinos.– Positrons are accompanied by neutrinos.



Beta Decay

• What new element is formed by the beta decay of carbon-14?

14 14 06 7 -1C N + + e

• The new element is nitrogen-14.• The electron is shown with a mass number of

zero and an atomic number of -1.– The total of the mass numbers and atomic numbers

are still equal.



Gamma Decay

• During alpha and beta decay, the nucleons left behind are often in an excited state.

• When returning to ground state, the nucleus emits electromagnetic radiation in the form of a gamma ray.

• The nucleus remains unchanged except for its energy state.



Types of Radioactive Decay




Visual Concept

Alpha, Beta, and Gamma Radiation



Nuclear Decay Series

• During nuclear decay, the daughter may be unstable as well, causing further decays.

• What element would be formed by thorium-232 undergoing 6 alpha and 4 beta decays?– Answer: lead-208



Classroom Practice Problems

• Find the missing item (X) in these reactions:

– Answer:

– Answer:

228 228 088 89 -1Ra Ac + + e

220 21686 84Rn Po + X

228 088 -1Ra X + e+

220 216 486 84 2Rn Po + He



Measuring Nuclear Decay

• The rate of decay is different for each nucleus.

N/t = -N– N is the number of nuclei, t is the time, and is the

decay constant. differs for every element.

– The rate of decay is called the activity.– The negative sign occurs because the number of

nuclei is decreasing.– SI unit: becquerel (Bq) or decays/s



Half-Life

• Half life is the time required for half of the nuclei to decay.– Half-lives can be very short (nanoseconds) or very long (millions

of years).

• Half-life is inversely related to the decay constant.



Half-Life

• Carbon-14 is radioactive with a half-life of 5715 years.

• The figure shows a decay curve for carbon-14. – Does the total number of nuclei

change?• No

– How much time has passed at T1/2?

• 5715 years– How much time has passed at

2T1/2?• 11 430 years

– How many blue circles will there be at 3T1/2 ?

• one



Radioactive Carbon Dating• All living things have about the same ratio of

carbon-14 to carbon-12.– Carbon-14 is radioactive, and carbon-12 is not. – After death, the ratio drops because the carbon-14

decays into nitrogen-14, while the carbon-12 is stable and remains.

– When the ratio is half the starting ratio, 5715 years have passed since death occurred.



Classroom Practice Problems

• A sample of barium-144 contains 5.0 10 9 atoms. The half-life is about 12 s. – What is the decay constant of barium-144?– How many atoms would remain after 12 s?– How many atoms would remain after 24 s?– How many atoms would remain after 36 s?

• Answers:– 0.058 s-1 – 2.5 109 atoms, 1.2 109 atoms, 6.2 108 atoms




Visual Concept

Half-Life




• Often scientists use radioactive carbon dating to determine the age of fossils. • What does the term radioactive mean?• Are all atoms radioactive?

• If not, how are the radioactive atoms different from those that are not radioactive?

• How can radioactivity be used to determine the age of a fossil?




8C describe the significance of mass-energy

equivalence and apply it in explanations of

phenomena such as nuclear stability, fission, and

fusion

8D give examples of applications of atomic and

nuclear phenomena such as radiation therapy,

diagnostic imaging, and nuclear power and

examples of applications of quantum phenomena

such as digital cameras



What do you think?

• Nuclear power and nuclear weapons are important and frequently-discussed issues in the world today.• How does a nuclear reactor produce energy?

• What is nuclear about it?

• What problems are associated with nuclear power?• Do atomic bombs and hydrogen bombs differ in the

way they produce energy?• If so, how are they different?



Nuclear Changes• For nuclear changes to

occur naturally, energy must be released.– Binding energy must

increase.– Lighter elements must

combine, and heavier elements must reduce in size.

– The greatest stability is for atoms with mass numbers between 50 and 60.



Fission• Fission occurs when a large nucleus absorbs a

neutron and splits into two or more smaller nuclei.

• Example of fission:

– It only occurs for heavy atoms. – The * indicates an an unstable state that lasts for

about a trillionth of a second.– X and Y can be different combinations of atoms that

have a total atomic number of 92.

1 235 236 *0 92 92n + U U X + Y + neutrons



Fission

• A typical fission reaction is shown above.• The products, Ba and Kr, have more binding energy than

the uranium. • As a result, energy is released.

– Each fission yields about 100 million times the energy released when burning a molecule of gasoline.

1 235 140 93 10 92 56 36 0n + U Ba + Kr + 3 n



Chain Reaction

• On the average, 2.5 neutrons are released with each fission.

• These neutrons are then absorbed and cause more fissions.– A chain reaction

occurs.




Visual Concept

Nuclear Fission



Nuclear Reactors

• Reactors manage the fission rate by inserting control rods to absorb some of the neutrons.

• Nuclear power plants and navy vessels use fission reactions as an energy source.– Reactors produce radioactive waste, and disposal is

one difficulty.– Presently 20% of the U.S. electric power is generated

by nuclear reactors.

• Atomic bombs use uncontrolled fission.



Fusion

• Light elements can combine and release energy as well.– Hydrogen atoms have less binding energy per

nucleon than helium atoms.

• Fusion is the source of a star’s energy.– Hydrogen atoms fuse to form helium atoms.– Much energy is released with each fusion.

• Hydrogen bombs use uncontrolled fusion.– First tested in 1952 but never used in war



Fusion as an Energy Source

• Fusion reactors are being developed.• Advantages of fusion reactors:

– The fuel source, hydrogen from water, is cheap.– The products of fusion are clean and are not

radioactive.

• Disadvantages of fusion:– It requires extremely high temperatures of roughly

108 K to force atoms to fuse.– It is difficult to keep the hydrogen atoms contained at

this temperature.




Visual Concept

Nuclear Fusion




• Nuclear power and nuclear weapons are important and frequently-discussed issues in the world today.– How does a nuclear reactor produce energy?

• What is nuclear about it?

– What problems are associated with nuclear power?– Do atomic bombs and hydrogen bombs differ in the

way they produce energy?

• If so, how are they different?




5A research and describe the historical development

of the concepts of gravitational, electromagnetic,

weak nuclear, and strong nuclear forces

5H describe evidence for and effects of the strong

and weak nuclear forces in nature



What do you think?

• When the idea of the atom was first conceived, it was thought to be a fundamental particle, indivisible and indestructible. We now know differently. • List every particle you can think of that is smaller

than an atom.• If you know the properties of these particles, list them as

well.• Which of the particles on your list are fundamental?



Fundamental Forces

• There are four fundamental interactions or forces in nature:– strong– electromagnetic– weak– gravitational

• They exert force using the exchange of mediating particles.– Photons are the mediating

particle exchanged between electrons.

• This causes repulsion.



Fundamental Forces

• Strong force– Holds protons and neutrons together in the nucleus

• Electromagnetic force– Creates forces between charged particles– Holds atoms and molecules together

• Weak force– A nuclear force that controls radioactive decay

• Gravitational force– The weakest force– Gravitons (the mediating particle) not yet discovered



Fundamental Forces



Classification of Particles

• All particles are classified as leptons, hadrons, or mediating particles.– Over 300 particles are

known.• Leptons are thought to be

fundamental.– Electrons are leptons.




• Hadrons are composed of smaller particles called quarks.– Quarks are thought to be

fundamental.– Protons and neutrons are

hadrons.• Two types of hadrons:

baryons and mesons• Hadrons interact through all

four of the fundamental forces, while leptons do not participate in strong force interactions.




• Protons and neutrons are baryons.

• What combination of up and down quarks would make a proton and a neutron?– Two up quarks (+4/3) and one

down quark (-1/3) gives a proton a charge of +1.

– One up quark (+2/3) and two down quarks (-2/3) gives a neutron a charge of zero.



Combinations of Quarks

• Baryons and mesons are distinguished by their internal structure.

• The particles above are a proton, a neutron, a pion, and a kaon.

• Mesons are unstable, and are not constituents of everyday matter.




Visual Concept

The Standard Model of Particle Physics



The Standard Model• The Standard Model is the current model used in particle physics.• How many fundamental particles are there in the standard model?

– Six quarks, six leptons, and an antiparticle for each (24 total)



Evolution of the Four Forces




Visual Concept

Quarks and their Charges




• When the idea of the atom was first conceived, it was thought to be a fundamental particle, indivisible and indestructible. We now know differently. • List every particle you can think of that is smaller than

an atom.• If you know the properties of these particles, list them as well.• Which of the particles on your list are fundamental?

• What are the four fundamental forces and what particle mediates each?

subatomic physicssection 1 © houghton mifflin harcourt publishing company preview section 1 the...

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