chapter 10 nuclear physics. copyright © houghton mifflin company 10-2 section 10.1: symbols of the...

Chapter 10Nuclear Physics

Copyright © Houghton Mifflin Company 10-2

Section 10.1: Symbols of the Elements

• An element is the fundamental atom by which all molecules are made.

• They are symbolized by their Latin names. Ex:

Copper Cu CuprumGold Au AurumIron Fe Ferrum

Sodium Na Natrium


Section 10.2: The Atomic Nucleus

• Atom consists of protons, neutrons, and electrons.

• The proton was discovered by Rutherford in 1918.

• The electron was discovered by J. J. Thomson in 1897.

• The neutron was discovered by James Chadwick in 1932.


The nucleus of the atom consists of protons and neutrons. They are nucleons.

The charge on proton = +e = 1.6x10-19 c

The charge on electron = -e = -1.6x10-19 c

The neutron has no net charge.


In the atom, each particle is allowed to take on integer spin, +/-1, +/-2,+/-3,… etc.

Positive for spin up, and negative for spin down.

Furthermore, each of these particles is thought to consist of even smaller particles, which are partly matter and partly energy, called quarks.


The proton, is thought to be made of two spin up quarks, each with a charge of +2/3e, and one spin down quark with a charge -1/3e.

Each quark has a charge of a fraction of the charge on the proton and electron, e.


The electron is thought to consist of three spin down quarks each of which has a charge of -1/3e.

The neutron is thought to consist of two spin down quarks and one spin quark.


Figure 10.2Rutherford's Alpha-Scattering Experiment

Alpha particles are helium nuclei


Nuclei of different elements are constructed by adding protons and neutrons to its nucleus.The atomic number is the number of protons in the nucleus.

The atomic mass is measured in atomic mass units (amu). 1 amu = (1/12) (mass of the C12 nucleus) For lighter nuclei, this is approximately equal to the number of protons + the number of neutrons.


Each element is then symbolized according to the following system:

)(amu)(in mass atomicnumber atomic mbolchemicalsy


Two nuclei that have the same atomic number are of the same element

Two nuclei that have different atomic numbers are of different elements.

Two nuclei that have the same atomic number but different atomic mass, are isotopes of each other.


Three isotopes of hydrogen are

Protium (1 proton, 0 neutrons),

Deuterium (1 proton, 1neutron), and

Tritium (1 proton, 2 neutrons)


Figure 10.5a: A Mass Spectrometer


Types of forces in nature:

a. The electromagnetic force: acts on magnets and electric charges. It has infinite range. Can be attractive or repulsive.

b. The gravitational force: acts on all matter. It is infinite in range, and is always attractive.


c. The strong force: Acts on nucleons. It is very strong, but its range is only of the size of the nucleus (10-

14m).

d. The weak force: Acts on all particles, but in particular the neutron. It is involved in beta decay, and its range is less than that of the strong force.


Relative strengths of forces outside the nucleus:

Strongest 1. Electromagnetic force

2. Gravity

3. Strong force (nonexistent)

Weakest 4. Weak force (nonexistent)


Relative strengths of forces within the nucleus.

Strongest 1. Strong force

2. Weak force

3. Electromagnetic force

Weakest 4. Gravity


Section 10.3: Radioactivity and Half-Life

• Radioactivity is the spontaneous decay of a parent nucleus into one or more daughter nuclei with the emission of a by-product nucleus. Example:

He Th - U 42

23490

23892

Parent daughter

By-product


In alpha decay, the by product is a helium nucleus. This type of decay is produced by a nucleus that is too big for its nucleons.

The atomic mass reduces by 4, and the atomic number by 2.


In beta decay, the by-product is an electron. In this type of decay, a neutron in the nucleus spontaneously disintegrates into a proton and an electron. This is produced by a nucleus that has too many neutrons relative to protons. The atomic number increases by 1, but the atomic mass stays the same. Example:

e N C- 01-

147

146


In gamma decay the by-product is a photon (gamma particle). This is produced by a nucleus that is just too energetic. As the nucleus releases its excess energy, a photon of the appropriate wavelength is produced. The atomic number and atomic mass do not change. Example:

Pb - Pb 20482

20482


In positron emission, the by-product is a positron. A positron is an electron with a positive charge, +e. It the antiparticle of the electron. This type of decay is produced when a proton in the nucleus of the parent spontaneously decays into a neutron, with the emission of a positron. The atomic number increases by one and the atomic mass stays the same. Example:

e O F- 01

178

179

Figure 10.7The Three Components of Radiation from Heavy Radionuclides

Radionuclides are the parents of decay

Figure 10.8The Decay of Uranium-238 to Lead 206

Figure 10.9A Plot of Number of Neutrons (N) Versus Number of Protons (Z) for the Nuclides


The half-life is

• the time taken for the number of parent nuclei to decrease to half of its initial value.

• the time taken for the number of daughter nuclei to equal the number of parent nuclei.

Figure 10.10The Decay Curve for Any Radionuclide

The decay rate =

time

decays of number the

= (constant) (#of parent nuclei)


Section 10.4: Nuclear Reactions

• Nuclear Reactions involve two or more nuclei of different elements to produce other elements.

• The first nuclear reaction was discovered by E. Rutherford.

H O - N He 11

178

147

42


The transuranium elements are elements with atomic number greater than that of uranium (92). They are stable, but must be synthesized by bombardment by alpha particles, or protons.


Section 10.5: Nuclear Fission

• Fission is the process in which a large nucleus is “split” into two or more intermediate-size nuclei, with the emission of neutrons, and the conversion of mass into energy. (SWT p. 242.)

n2 Xe Sr U 10

14054

9038

23692


A nuclear chain reaction is a triggered chain of events leading to an uncontrolled release of neutrons and a large amount of energy.

Figure 10.15Subcritical and Supercritical Masses

When the flux of neutrons is just enough to start the chain reaction, the system is said to be at critical mass.


A nuclear reactor is a controlled nuclear chain reaction.


Section 10.6: Nuclear Fusion

• Nuclear fusion is the process in which smaller combine to form larger ones, with the release of energy. (SWT p. 246)

• The deuterium-tritium reaction:

n HeH H 10

42

31

21


The binding energy

= the amount of energy needed to pull the nucleus apart.

= (mass lost)c2

Stable nuclei tend to be toward higher binding energy.


Figure 10.20The Relative Stability of Nuclei

chapter 10 nuclear physics. copyright © houghton mifflin company 10-2 section 10.1: symbols of the...

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mass spectrometer slide

number of neutrons

different atomic mass

atomic nucleus atom

nuclear physics slide

number of protons

spin quark

net charge