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Nuclear Chemistry

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Page 1: Nuclear Chemistry. The Nucleus Remember that the nucleus is comprised of protons and neutrons. The number of protons is the atomic number. The number

Nuclear Chemistry

Page 2: Nuclear Chemistry. The Nucleus Remember that the nucleus is comprised of protons and neutrons. The number of protons is the atomic number. The number

The Nucleus

• Remember that the nucleus is comprised of protons and neutrons.

• The number of protons is the atomic number.• The number of protons and neutrons together is

the mass of the atom.

Page 3: Nuclear Chemistry. The Nucleus Remember that the nucleus is comprised of protons and neutrons. The number of protons is the atomic number. The number

Isotopes

• Not all atoms of the same element have the same mass due to different numbers of neutrons in those atoms.

• There are three naturally occurring isotopes of uranium:– Uranium-234– Uranium-235– Uranium-238

Page 4: Nuclear Chemistry. The Nucleus Remember that the nucleus is comprised of protons and neutrons. The number of protons is the atomic number. The number

Stable NucleiThe shaded region in the figure shows what nuclides would be stable, the so-called belt of stability.

http://phet.colorado.edu/en/simulation/isotopes-and-atomic-mass

Most nuclei are stable.

It is the ratio of neutrons to protons that determines the stability of a given nucleus.

Page 5: Nuclear Chemistry. The Nucleus Remember that the nucleus is comprised of protons and neutrons. The number of protons is the atomic number. The number

Radioactivity

• It is not uncommon for some nuclei to be unstable, or radioactive.

• There are no stable nuclei with an atomic number greater than 83.

• Radioisotopes = isotopes that are unstable and thus radioactive

• There are several ways radionuclides can decay into a different nuclide.

Page 6: Nuclear Chemistry. The Nucleus Remember that the nucleus is comprised of protons and neutrons. The number of protons is the atomic number. The number

Radioactive Series

• Large radioactive nuclei cannot stabilize by undergoing only one nuclear transformation.

• They undergo a series of decays until they form a stable nuclide (often a nuclide of lead).

• Transmutation = the reaction by which the atomic nucleus of one element is changed into the nucleus of a different element

Page 7: Nuclear Chemistry. The Nucleus Remember that the nucleus is comprised of protons and neutrons. The number of protons is the atomic number. The number

pHET simulations of alpha decay of Polonium-211 to form Lead-207 and of

Beta decay of Hydrogen-3 to Helium-3

• http://phet.colorado.edu/en/simulation/alpha-decay

• http://phet.colorado.edu/en/simulation/beta-decay

Page 8: Nuclear Chemistry. The Nucleus Remember that the nucleus is comprised of protons and neutrons. The number of protons is the atomic number. The number

Types of Radioactive Decay Alpha Decay

= Loss of an -particle (a helium nucleus)

He42

U238

92 Th234

90 He42+

CorrectionAtomic # decreases by 2# of protons decreases by 2# of neutrons decreases by 2Mass # decreases by 4

Page 9: Nuclear Chemistry. The Nucleus Remember that the nucleus is comprised of protons and neutrons. The number of protons is the atomic number. The number

Types of Radioactive Decay Beta Decay

= Loss of a -particle (a high energy electron)

0

−1 e0

−1or

I131

53 Xe131

54 + e0

−1

Atomic # increases by 1# of protons increases by 1# of neutrons decreases by 1Mass # remains the same

Page 10: Nuclear Chemistry. The Nucleus Remember that the nucleus is comprised of protons and neutrons. The number of protons is the atomic number. The number

Types of Radioactive Decay Positron Emission

= Loss of a positron (a particle that has the same mass as but opposite charge than an electron)

e01

C11

6 B11

5 + e01

Atomic # decreases by 1# of protons decreases by 1# of neutrons increases by 1Mass # remains the same

Page 11: Nuclear Chemistry. The Nucleus Remember that the nucleus is comprised of protons and neutrons. The number of protons is the atomic number. The number

Types of Radioactive Decay Gamma Emission

= Loss of a -ray (a photon of high-energy light that has no mass or charge & that almost always accompanies the loss of a nuclear particle; often not shown when writing nuclear equations)

00

Page 12: Nuclear Chemistry. The Nucleus Remember that the nucleus is comprised of protons and neutrons. The number of protons is the atomic number. The number
Page 13: Nuclear Chemistry. The Nucleus Remember that the nucleus is comprised of protons and neutrons. The number of protons is the atomic number. The number

Artificial Transmutation= done by bombarding the nucleus with high-energy particles (such

as a neutron or alpha particle), causing transmutation

4020Ca + _____ -----> 40

19K + 11H

9642Mo + 2

1H -----> 10n + _____

**Natural transmutation has a single nucleus undergoing change, while artificial transmutation will have two reactants (fast moving particle & target nuclei.**

Page 14: Nuclear Chemistry. The Nucleus Remember that the nucleus is comprised of protons and neutrons. The number of protons is the atomic number. The number

Nuclear Fission• Nuclear fission is the type of reaction carried out in

nuclear reactors.• = splitting of large nuclei into middle weight nuclei and

neutrons

Page 15: Nuclear Chemistry. The Nucleus Remember that the nucleus is comprised of protons and neutrons. The number of protons is the atomic number. The number

Nuclear Fission

• Bombardment of the radioactive nuclide with a neutron starts the process.

• Neutrons released in the transmutation strike other nuclei, causing their decay and the production of more neutrons.

• http://phet.colorado.edu/en/simulation/nuclear-fission

• This process continues in what we call a nuclear chain reaction.

Page 16: Nuclear Chemistry. The Nucleus Remember that the nucleus is comprised of protons and neutrons. The number of protons is the atomic number. The number

• = the combining of light nuclei into a heavier nucleus

• 21H + 2

1H 42He + energy

• Two small, positively-charged nuclei smash together at high temperatures and pressures to form one larger nucleus.

Nuclear Fusion

Page 17: Nuclear Chemistry. The Nucleus Remember that the nucleus is comprised of protons and neutrons. The number of protons is the atomic number. The number

Energy changes in Nuclear Reactions E =mc2

• Einstein E =mc2

• mass defect For nuclear reactions

• E = energy in Joules (J = kg•m2/s2)

• m = mass in kg• C = speed of light • (2.9979 x 108 m/s)

Page 18: Nuclear Chemistry. The Nucleus Remember that the nucleus is comprised of protons and neutrons. The number of protons is the atomic number. The number

Half-Life

= the time it takes for half of the atoms in a given sample of an element to decay

- Each isotope has its own half-life; the more unstable, the shorter the half-life.

- Table T Equations:

fraction remaining = (1/2)(t/T)

# of half-lives remaining = t/T

Key: t = total time elapsed T = half-life

Page 19: Nuclear Chemistry. The Nucleus Remember that the nucleus is comprised of protons and neutrons. The number of protons is the atomic number. The number

PhET simulation of decay and half-life

• http://phet.colorado.edu/en/simulation/radioactive-dating-game

Page 20: Nuclear Chemistry. The Nucleus Remember that the nucleus is comprised of protons and neutrons. The number of protons is the atomic number. The number

Sample Half-Life Question 1AMost chromium atoms are stable, but Cr-51 is an unstable isotope with a half-life of 28 days. (a) What fraction of a sample of Cr-51 will remain after 168 days?

Step 1: Determine how many half-lives elapse during 168 days.

Step 2: Calculate the fraction remaining.

Page 21: Nuclear Chemistry. The Nucleus Remember that the nucleus is comprised of protons and neutrons. The number of protons is the atomic number. The number

Sample Half-Life Question 1B

(b) If a sample of Cr-51 has an original mass of 52.0g, what mass will remain after 168 days?

Step 1: Calculate the mass remaining:

mass remaining = fraction remaining X original mass

(Note: Mass remaining can also be calculated by dividing the current mass by 2 at the end of each

half-life.)

Page 22: Nuclear Chemistry. The Nucleus Remember that the nucleus is comprised of protons and neutrons. The number of protons is the atomic number. The number

Sample Half-Life Question 2

How much was present originally in a sample of Cr-51 if 0.75gremains after 168 days?

Step 1: Determine how many half-lives elapsed during 168 days.

Step 2: Multiply the remaining amount by a factor of 2 for each half-life.

Page 23: Nuclear Chemistry. The Nucleus Remember that the nucleus is comprised of protons and neutrons. The number of protons is the atomic number. The number

Some practical uses of Radioisotopes (dating, chemical tracers, industrial applications, medical

applications, nuclear power plants)

Medical Uses• 60Co (cobalt-60) used in

cancer treatments and used to kill bacteria in food products

• 226Ra (Radium-226) used in Cancer treatment

• 131I diagnosis and treatment of thyroid disorders

• 11C Positron emission tomography (PET scans)

Other Uses• 14C archaeological dating (of

once living things) and radiolabelled organic compounds

• 238U archaeological dating (U-238 to Pb-206 ratio)

• 241Am (Americium-241) smoke detectors

• 235U nuclear reactors and weapons