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Chapter 25. Nuclear Chemistry. Nuclear Radiation. Nuclear chem the study of the structure of atomic nuclei and the changes they undergo. No e- / orbitals No e- sharing or transferring No cpds formed No bondings. The Discovery of Radioactivity.

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Chapter 25

Chapter 25

Nuclear Chemistry


Nuclear Radiation

  • Nuclear chem

  • the study of the structure of atomic nuclei and the changes they undergo.

  • No e- / orbitals

  • No e- sharing or transferring

  • No cpds formed

  • No bondings


The Discovery of Radioactivity

  • In 1895, Roentgen found that invisible rays were emitted when e- bombarded the surface of certain materials.

  • they caused photographic plates to darken.

  • named these invisible high-energy emissions X rays.


The Discovery of Radioactivity

  • At that time, French physicist Becquerel was studying minerals that

  • emit light after being exposed to sunlight (phosphorescence).

  • Building on Roentgen’s work, Becquerel wanted to determine whether phosphorescent minerals also emitted X rays.


The Discovery of Radioactivity

Becquerel accidentally discovered that phosphorescent U salts – even when not exposed to light – produced spontaneous emissions that darkened photographic plates.


The Discovery of Radioactivity

Marie Curie (1867–1934) and her husband Pierre (1859–1906) took Becquerel’s mineral sample and isolated the components emitting the rays.


The Discovery of Radioactivity

  • Conclusion:

  • the darkening of the photographic plates was due to rays emitted specifically from the U atoms in the mineral sample.

  • named the process by which materials give off such rays radioactivity;

  • the rays and particles emitted by a radioactive source are called radiation.


Types of Radiation

  • Isotopes

  • are atoms of the same element that have different #s of n0.

  • Radioisotopes

  • Isotopes of atoms with unstable nuclei ready to emit radiations.

  • unstable nucleiis due to big diffce in the # of

  • p+and n0. e.g.U has 92 p+and over 140 n0

    • Mg has 12 p+and 12 n0(stable)


Quick write
Quick write

What kind of atoms are radioactive?


Types of Radiation

  • radioactive decay

  • A process that unstable nuclei emit radiation to attain more stable atomic configns.

  • During radioactive decay, unstable atoms lose energy by emitting 1 of several types of radiation.


Types of radiation
Types of Radiation

3 common types of radiation

  • alpha (α)

  • beta (β)

  • gamma (γ)


2

-1

0

1837


Quick write1
Quick Write

What are α particles? How many p+, n0 and e- does each particle carry?


Radioactive decay
Radioactive Decay

Radioactive Decay

  • unstable nuclei loses energy by emitting radiation: ‘particles’ or ‘energy (non-particles)’.

  • become lighter

  • Natural (not human initiated; can’t stop or slow down)

  • not requiring any energy input.

  • Spontaneous


Quick write2
Quick Write

What are the areas that α decay and β decay have in common? (4 bullet pts)



Half life
Half-life

Half-life

  • Time required for one half of the nuclei of a radioisotope sample to decay.

  • e.g. U → Th + α-particle

  • Each radioisotope has a characteristic t 1/2


Half life1
Half-life

Half-life

  • Time required for one half of the nuclei of a radioisotope sample to decay.

  • e.g. U → Th + α-particle

    238U → 234Th + 4He

  • Each radioisotope has a characteristic t 1/2

92

90

2



Effect of an Electric Field (1)

  • What radiations are deflected toward the -ve plate? Why?

  • What radiations are deflected toward the +velyplate? Why?


Quick write3
Quick Write

What is radioactive decay? (5 bullet points)


Quick write4
Quick Write

Describe and explain the paths of α, β, and γ radiation under the influence of electric field?


Effect of an electric field 2
Effect of an Electric Field (2)

  • The +vely charged α particles are deflected towards the -ve plate.

  • The –vely charged β particles are deflected towards the +ve plate.

  • the neutral γ radiation travels in a straight line.


Quick write5
Quick Write

What are β particles? Do they carry any charges?


Quick write6
Quick Write

What is γ radiation? What charge does it carry?


Effect of an Electric Field (2)

  • β particles are deflected towards the +ve plate.

  • β particles undergo greater deflection because →less mass.


Effect of an Electric Field (3)

  • γ ray, (no electrical charge), are not deflected.


Types of Radiation- α radiation

  • An α particle has the same composition as a He nucleus—2 p+and 2 n —

  • 2+ due to the presence of the 2 p+.


Types of Radiation- α radiation

  • α radiation—a stream of αparticles.

  • Ra-226, (88 p+and 138 n0), undergoes α decay by emitting an α particle.


Types of Radiation- α radiation

  • After the decay, the resulting atom has an atomic # of 86, a mass # of 222.

  • The new radiosiotope is Rn-222.


Types of Radiation

  • The particles involved are balanced.

  • i.e. the sum of the mass #s (superscripts) = the sum of the atomic #s (subscripts) on each side of the arrow.


Types of Radiation

  • Because of their mass and charge, α particles are relatively slow-moving compared with other …

  • Thus, α particles are not very penetrating—a single sheet of paper stops.


Types of Radiation—β Radiation

  • A β particle is a very-fast moving e- that has been emitted from a n0of an unstable nucleus.

  • β particles are represented by the symbol . The ‘0’ superscript indicates the insignificant mass of an e- in comparison with the mass of a nucleus.


Types of Radiation—β Radiation

  • The –1 subscript denotes the -ve charge of the particle.

  • β radiation consists of a stream of fast-moving e-.


Types of Radiation

An example of the β decay process is the decay of I-131 into Xe-131 by β-particle emission.


Types of Radiation-- β Radiation

  • The mass # of the product nucleus is the same as that of the original nucleus (both 131), but its atomic # has increased by 1 (54 instead of 53).


Types of Radiation-- β Radiation

  • This change in atomic #, and thus, change in identity, occurs because the e- emitted during the β decay has been removed from a n0, leaving behind a p+.


Types of radiation radiation
Types of Radiation—β Radiation

β radiation:

fast-moving e- formed by decomposition of a n0in an atom.

Quick write: why the mass # remain 14 while there is an additional p+?


Types of Radiation

  • Because β particles are both lightweight and fast moving, they have greater penetrating power than α particles.

  • A thin metal foil is required to stop β particles.


Types of Radiation

  • γ rays are high-energy (short wavelength) electromagnetic radiation.

  • They are denoted by the symbol .

  • Both the subscript and superscript are ‘0’.


Types of Radiation

  • Thus, the emission of γ rays does not change the atomic # or mass # of a nucleus.

  • γ rays almost always accompany α and β radiation, as they account for most of the energy loss that occurs as a nucleus decays.


Types of Radiation

  • e.g. γ rays accompany the α-decay rxn of U-238.

  • The 2 in front of the γ symbol indicates that 2 γ rays of different frequencies are emitted.

  • Because γ rays have no effect on mass # or atomic #, it is customary to omit them from nuclear eqns.


Radioactive Decay

  • Of all the known isotopes, only about 17% are stable and don’t decay spontaneously.


Beta Decay

  • A radioisotope that lies above the band of stability is unstable because it has too many n relative to its # of p+.

e.g. unstable has a n0 / p+ ratio of 1.33 : 1, whereas stable elements of similar mass, such as

and , have n0 / p+ ratios ≈1:1


Beta Decay

It is not surprising then that undergoes beta decay, as this type of decay decreases the # of n0 in the nucleus.

Note that the atomic # of the product nucleus, , has increased by 1.

  • The N-14 atom now has a stable n0 / p+ratio of 1 : 1.


Beta Decay

  • Thus, β emission has the effect of increasing the stability of a n0 -rich atom by lowering its n0 / p+ratio.

  • The resulting atom is closer to, if not within, the band of stability.


Alpha Decay

  • All nuclei with more than 83 p+are radioactive and decay spontaneously.

  • Both the # of n0and the # of p+must be reduced in order to make these radioisotopes stable.

  • These very heavy nuclei often decay by emitting α particles.


Alpha Decay

  • e.g. , Po-210 spontaneously decays by α emission.

The atomic # of decreases by 2 and the mass # decreases by 4 as the nucleus decays into .


Nuclear fission
Nuclear Fission

Nuclear Fission

  • Nuclei of certain isotopes (e.g U-238) are bombarded with n0split into smaller fragments of similar sizes.

Unleash enormous amt of energy e.g. 1 kg of U- 235 (explosion of 20000 tons of dynamite)




Nuclear fission3
Nuclear Fission

  • In a chain rxn

  • some of the n0 produced react with other fissionable atoms

  • producing more n0 which react with still more fissionable atoms.


Nuclear fission4
Nuclear Fission

A Nuclear Power Plant



Nuclear fission5
Nuclear Fission

Neutron Moderation

  • a process that slows down n0 so the reactor fuel (U-235 or Po-239) captures them to continue the chain rxn.


Nuclear fission6
Nuclear Fission

Neutron Absorption

  • a process that decreases the # of slow-moving n0.

  • water slow down the n0 in the reactor

  • Control rods—made of a material such a Cd, or B, are used to absorb n0.


Nuclear fission7
Nuclear Fission

Nuclear Fission

  • Nuclei of certain isotopes are bombarded with neutrons

  • split into smaller fragments of similar sizes.

  • Unleash enormous amt of energy

  • e.g. 1 kg of U- 235 ( explosion of 20000 tons of dynamite)


Nuclear fission8
Nuclear Fission

Uncontrolled fission

  • Total energy release is instantaneous (fraction of a second);

  • uncontrolled chain rxn—atomic bomb;

Controlled fission

  • so energy is released more slowly (nuclear power)


Controled uncontroled fission
Controled/uncontroled fission


Fission and fusion of atomic nuclei
Fission and Fusion of Atomic Nuclei

  • The sun is not actually burning (combustion).

  • would have burned out approximately 2000 years after it was formed.


Nuclear fusion
Nuclear Fusion

Nuclear Fusion

  • Occur when small nuclei combine to produce a nucleus of greater mass

  • H nuclei (p+) fuse to make He nuclei


Nuclear fusion1
Nuclear Fusion

  • much more energy than fission rxn

  • occur at very high temp (40, 000, 000 ° C)

  • good energy source--- cheap fuel;

  • readily available; controlled


Nuclear fusion2
Nuclear Fusion

  • problems; high temp to initiate? --- needs an atomic bomb to trigger a nuclear fusion rxn…

  • **** the energy release per g of the material is much larger in nuclear fusion or fission rxns than in chemrxns.

  • change in mass (calculated by E = mc2 ) is small but significant in nuclear rxns.


Quick write7

25.3

Quick-write

How do fission rxns and fusion rxns differ?


Nuclear fusion3
Nuclear Fusion

Fusion

  • occurs when nuclei combine to produce a nucleus of greater mass.

  • In solar fusion, H nuclei (p+) fuse to make He nuclei and 2 positrons.



Nuclear fusion5
Nuclear Fusion

Fusion rxns

  • small nuclei combine,

  • release much more energy than fission rxns (large nuclei split).


Nuclear fusion6

25.3

Nuclear Fusion

  • The use of controlled fusion as an energy source on Earth is appealing.

  • The potential fuels are inexpensive and readily available.

The problems with fusion

  • in achieving the high temperatures necessary to start the rxn and

  • in containing the rxn once it has started.


Applications of nuclear power
Applications of Nuclear Power

Cancer treatment (radiation therapy)

  • high-energy X-rays are directed at a person’s body to kill cancer cells

    X-rays, MRI, CT scan

  • e.g. dentist’s office “radiation therapy”

    Nuclear medicine

  • Uses radionuclides in the diagnosis of disease

  • Relies on process of radioactive decay


CST example 1

A 2-cm thick piece of cardboard placed over a radiation source would be most effect in protecting against which type of radiation?

A alpha

B beta

C gamma

D x-ray


CST problem 2

Which equation correctly represents the alpha decay of polonium-214?

A 214

Po 214 0

84 85



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