1. 31.1 Structure and Properties of the Nucleus
Atomic mass
protons + neutrons
Chemical symbol
Atomic number
Protons
Number of neutrons = A - Z
The size of the nucleus

2. From the following table, you can see that the electron is considerably less massive than a nucleon.

3. There are Four Fundamental Forces:
These are responsible for all we see accelerate
1) The Electromagnetic Force
2) The Gravitational Force
These act over a very small range
3) The Strong Nuclear Force, 31.2
© Laura Fellman
4) The Weak Nuclear Force, 31.5

4. The Unification of Forces
Physicists would love to be able to show someday that the four fundamental forces are actually the result of one single force that was present when our universe began.
© Laura Fellman
Superstring Theory is an interesting and promising possibility in this quest:
Web Links: Superstring Theory The Elegant Universe The Fabric of the Cosmos

5. Mass is a form of energy related by
Nucleus (or nuclei) ~ center bit of the atom
Nucleons ~ protons & neutrons (bits inside the nucleus)
Z – Atomic # (p’s)
A – Mass # (p’s & n’s)
Mass is a form of energy related by
In nuclear reactions
mass – energy is conserved

7. What are eV (electron volts) and its units ?
Is a small energy unit, used in atomic and nuclear processes.
1 eV = 1.6 x10-19J
1 MeV = 1.6 x10-13J
Is the energy given to an electron by accelerating it through 1 volt of electric potential difference…
More on this later…
What is an amu ?
is the standard unit that is used for indicating mass on an atomic scale. It is defined as one twelfth of the mass of an unbound neutral atom of carbon-12
1 atomic mass unit (amu) = x kg
1 amu = MeV (E =mc2) mass – energy equivalence!

8. Mass Deficit & Binding Energy
In order to break up a nucleus into separate nucleons, you must overcome a strong but short range ‘Nuclear Force’ that binds / attracts the nucleons together.
You need to provide energy to overcome this force.
The energy required to separate / break up the nucleus is referred to as the binding energy and corresponds to the mass deficit
This explains why the mass of the individual nucleons is heavier.

9. Just like energy is needed to pull a bucket of water out of a well against gravity…
…energy is also required to “pull” nucleons out of a nucleus.
i.e. energy is needed to break apart a nucleus.
Binding Energy

10. Binding Energy and Nuclear Forces
An Alpha particle P N P N
The total mass of a stable nucleus is always less than the sum of the masses of its separate protons and neutrons.

11. This difference between the total mass of the constituents and the mass of the nucleus is called the total binding energy of the nucleus.
Binding energy
Energy required to separate nuclei

12. Problems
a) Calculate the binding energy, and binding energy per nucleon for a nucleus of
The nucleus contains 7 protons and 7 neutrons.
Binding energy = Δmc2
Binding energy
Binding energy per nucleon =

13. b) Calculate the binding energy, and the binding energy per nucleon, for
The nucleus contains 82 protons and 126 neutrons.
Binding energy = Δmc2
Binding energy
Binding energy per nucleon =

14. Binding Energy per nucleon
Binding energy per nucleon = binding energy of nucleus / # of nucleons
The stability of a nucleus depends not so much on the total binding energy of the atom but due to the atom’s binding energy per nucleon.
Greater the binding energy per nucleon, the more stable the nucleus (ie Fe).
Mass number 50
100
150
200
B.E per nucleon
(MeV) 2 4 6 8
56Fe
238U
4He
7Li

15. What’s going on inside the nucleus?
The nucleus is held together by a very short-range attractive force (the Nuclear Force) that exist between nucleons.  
This force is greater than the long range repulsive Electrostatic (Coulomb) force that exist between all the positive protons in the nucleus.
Nuclear force similar to "Velcro,“ i.e. only acts as a force if in close proximity (less than m).
Nuclides with large numbers of protons need more neutrons (which only exert the attractive strong nuclear force) to overcome the electric repulsion between protons. For these very large nuclei, ‘extra’ number of neutrons can overcome the electric repulsion between protons.
Elements with Atomic numbers greater then 82 (Lead) are basically too big (wide), the electrostatic forces are too much for the Nuclear force and the element will under go Radioactive decay (via ,  and or ).

18. The higher the binding energy per nucleon, the more stable the nucleus.
More massive nuclei require extra neutrons to overcome the Coulomb repulsion of the protons in order to be stable.
The force that binds the nucleons together is called the strong nuclear force. It is a very strong, but short-range, force. It is essentially zero if the nucleons are more than about m apart.

19. Which nucleus has the largest binding energy per nucleon?
56Fe
116Sn
194Pt
238U
4He
35Cl
56Fe
116Sn
194Pt
238U

20. Which of the following nuclei has the lowest rest mass per nucleon
Which of the following nuclei has the lowest rest mass per nucleon. For 35Cl that number would be MCl/35.
4He
35Cl
56Fe
116Sn
194Pt
238U
4He
35Cl
56Fe
116Sn
194Pt
236U

21. Average binding energy per nucleon averages over nucleons near the center that are more tightly bound and nucleons further away from the center that may be almost unbound. The largest semi-stable nucleus (it has a half life of 1.2 seconds) is Ununpentium (~286Uup113). There are no larger nuclei because any additional added nucleons would be unbound. This is because:
Coulomb forces are long range and nuclear forces are short range.
Coulomb forces are short range and nuclear forces are long range.
Both coulomb and nuclear forces are short range.
Both coulomb and nuclear forces are long range.
None of the above

22. 500Uup113 cannot exist because:
The neutron’s magnetic moments would repel each other.
There is no way to fabricate such an unbalanced nucleus.
The additional neutrons would have to be stacked into unbound higher energy levels.
None of the above.
All of the above.