nuclear chemistry

1. Chapter 21:
Nuclear Chemistry
1

2. 2
Some basics
• A nucleus contains protons (p) and neutrons (n); bound together by strong
nuclear force
• Particles in the nucleus are collectively called nucleons
• Nucleus is tiny part of entire atom; Rnucleus ≈ 10–15 m, Ratom ≈ 10–10 m
A = mass number = number of nucleons (p + n)
Z = atomic number = number of protons (p)
or
• An atom can be symbolized as:

3. 3
Subatomic particles
The nucleus of an atom can undergo decay to produce a new nucleus. Below
are some other possible decay products.
Subatomic particles:
Proton
Neutron
Electron (β– particle)
Positron (β+ particle)
Like electron but
with positive charge
Other decay products:
Gamma ray No charge and no mass; high energy
electromagnetic radiation
Alpha (α) particle
Made up of two protons and two
neutrons

4. 4

5. 5
Nuclear stability
Of the thousands of nuclides that exist, about 250 are stable (blue). All others
are unstable (green).
1
2
3
Graph shows neutron/proton ratio of all
known isotopes.
• Light atoms have n/Z ratio of 1:1
• Large atoms are about 1.5:1
If nuclei have n/Z ratio in region:
too many neutrons/not enough
protons
too many protons/not enough
neutrons
too many protons and neutrons
1
2
3

6. 6
Stable nuclear isotopes
Currently known isotopes
• Most stable isotopes have an even number of protons and neutrons.
• There are slightly more stable isotopes with even number of protons and
odd number of neutrons.
• Isotopes with an odd number of protons and neutrons are generally not
stable.

7. 7
Radioactive decay processes
• Nuclei can be unstable (too much mass, energy, etc.)
• Unstable nuclei become stable via radioactive processes
• Conversion of one nuclide into another = “nuclear transmutation”
• Radioactivity is the emission of radioactive decay products
Five types of radioactive decay
1.) Alpha (α) decay – a particle containing two protons and two neutrons
(called an α particle) is emitted
These are called nuclear equations.

8. 8
Radioactive decay processes
2.) Beta (β–) decay – an electron (β– particle) is emitted; atomic number is
increased by 1
General form:
Note the conservation of mass
3.) Positron emission (β+ decay) – emission of a positron (β+ particle) from the
nucleus; atomic number is decreased by 1 and a neutron is gained
General form:
Note the conservation of mass

9. 9
Radioactive decay processes
4.) Gamma (γ) ray emission – emission of gamma radiation; high energy (short
wavelength) photons; emitted from excited nuclei or with other decay processes
Note: gamma rays are emitted with most radioactive decay processes;
commonly left out of nuclear equations
Note the conservation of mass
5.) Electron capture – occurs when an inner electron in an atom is captured by
the atom’s nucleus and a proton is converted to a neutron; atomic number
decreased by 1
Note the conservation of mass
General form:

10. 10
Radioactive decay processes

11. 11
Nuclear binding energy
• The energy required to bind the nucleons in a nucleus together; usually given
as the amount of energy per nucleon.
• Larger binding energies generally mean a more stable nucleus
• Measured in MeV or MeV/nucleon (where eV is electron volts)
Example:
The total binding energy of an alpha particle nucleus is 28.4 MeV. The binding
energy per nucleon is

12. 12
Separating a nucleus requires energy
Helium nucleus Constituent nucleons
n
p+
n
p+
n
n p+ p+
Energy +
Binding nucleons together produces energy
Constituent nucleons
n n p+ p+
Helium nucleus
p+
n
p+
n
+ Energy

13. 13
Mass defect
• The mass of a nucleus is smaller than the masses of all its constituent
nucleons.
• This ‘missing mass’ is called the mass defect
Mass:
Constituent nucleons
n n p+ p+
Helium nucleus
p+
n
p+
n
+ Energy
4.0331 amu 4.0026 amu
The missing mass is the energy that was released when the nucleus was
formed. This missing mass is the nuclear binding energy!
mass defect = 0.0305 amu

14. 14
Mass defect
Albert Einstein’s famous mass-energy equivalence equation1 relates mass with
energy.
Don’t forget the conversion factors (J → eV → MeV)
c = speed of light (3.0×108 m s–1)
E = energy (in J)
m = mass in kg
The nuclear binding energy is directly related to the missing mass (i.e. mass
defect). Adding energy increases mass while reducing energy reduces mass.

15. 15
Example
Carbon-16 has a mass defect of 0.11888 amu. What is the nuclear binding
energy of this isotope (in MeV/nucleon)?
Determine number of nucleons
mass number = p + n = 16 # nucleons = 16
Convert mass (amu to kg) 1 amu = 1.66x10–27 kg
Get nuclear binding energy (in MeV)

16. 16
Example
Carbon-16 has a mass defect of 0.11888 amu. What is the nuclear binding
energy of this isotope (in MeV/nucleon)?
Get nuclear binding energy per nucleon
# nucleons = 16
A: 6.93 MeV/nucleon

17. 17
Fission
Nuclear fission is splitting a nucleus into smaller components
Here, a neutron is absorbed by a massive uranium-235 nucleus becoming the
unstable uranium-236.
The unstable nucleus quickly splits, here, into barium-141 and krypton-92. Three
neutrons are released as well as energy (in the form of heat).

18. 18
Example: Energy from fission (Part 1)
The following fission reaction results in a mass defect of 0.18489 amu. How
much energy (in J) was released when one uranium-235 undergoes fission?
Convert amu to kg
Get energy
A: 2.762×10–11 J

19. 19
Example: Energy from fission (Part 2)
How much energy (in kJ mol–1) is released when one mole of uranium-235
undergoes fission?
Convert energy into kJ mol–1
https://www.youtube.com/watch?v=pwS6Bq3ruWA

20. 20
Fusion
Nuclear fusion is the process of combining lighter nuclei into heavier nuclei. The
resulting mass defect results in the release of enormous amounts of energy.
The sun undergoes fusion reactions 24/7 and produces energy on a galactic level.

21. 21
Kinetics of radioactive decay
All radioactive decay processes follow 1st order kinetics
Rate law:
Integrated rate law:
Half-life:
N = number of
radioactive atoms
(mass, # atoms,
moles, etc.)
Cobalt-60 has a half life of 5.27 years.

22. 22
Example
Cobalt-60 decays with a half-life of 5.27 years to produce nickel-60. What is the
decay constant (in yr–1) for Cobalt-60?
A: 0.132 yr–1

23. 23
Example
What fraction of a sample of cobalt-60 will remain after 15 years?
A: 0.138

24. 24
Example
How long does it take (in yr) for a sample of cobalt-60 to disintegrate to the
extent that only 2.0% of the original amount remains?
A: 0.138
Time so 1st-order integrated rate law
Final number of Co-60 is 2% of original sample
Solve for t

25. 25
21 Practice
How long does it take (in yr to one decimal place) for a sample of cobalt-60 to
disintegrate to the extent that only 10.0% the original amount remains? The half-
life of cobalt-60 is 5.26 years. A:
A copper-63 nucleus has a mass defect of 0.59223 amu. What is the nuclear binding
energy of this isotope (in MeV and MeV/nucleon each to two decimal places).
A: