Nuclear Reactions

1. Unified atomic mass unit. u = 1.6605402 x 10-27kg. 1/12 the mass of an atom of carbon-12.

2. Mass Defect = total mass of nucleons - mass of nucleus. Mass nucleus = Mass atom - Z(mass of electron)

(Mass of protons + mass of neutrons) > mass of nucleus.

Example Q2


3. Why is there a difference? and where is the extra mass?

Einstein's Mass-Energy formula.

E = mc2, this basically states that mass and energy are equivalent and can be converted into one another.

So the mass defect of a nucleus has been converted into energy and is stored in the nucleus ... called binding energy (Eb)

Eb= (mass defect)(c2) ... Examples Q3 and Q4.

This is the work (energy) required to completely separate the nucleons of a nucleus. The greater the Eb per nucleon the more stable the nucleus. Most nuclei have a binding energy per nucleon of approximately 8 MeV.

NOTE: The binding energy is not something a nucleus has - it is energy it lacks relative to the total mass of its separate parts.

4. Energy released in a decay.

If mass before > mass after (combined) then energy is released.

So for decay to take place, [binding energy/nucleon of decaying nucleus] < [binding energy/nucleon of produced nuclei].

5. Nuclear reactions. Occur when a given nucleus is struck by another nucleus, or by a simpler particle such as a gamma ray or neutron, so that an interaction takes place (Giancoli). This results in a transmutation or the transformation of one atom into another.

1934-36: Fermi produced two new elements, neptunium and plutonium, by bombarding urnanium with neutrons. These elements are heavier than uranium.

1938: Hahn and Strassmann discovered that this process sometimes produced smaller nuclei that were half the size of the original urnanium nucleus. Meitner and Frisch realized that after absorbing a neutron the uranium nucleus had split in half. This was the start of nuclear fission.

6. Nuclear fission. Heavy nucleus splits into lighter nuclei. If a neutron is added to the nucleus, it then splits into two smaller nuclei and 2-3 neutrons. These neutrons can be used to create a chain reaction with other nuclei. The key is to control the chain reaction.

Nuclear Power

7. Nuclear fusion. Joining of two light nuclei into a heavier one (with less overall mass).

8. Fusion in Stars needs high temp and high pressure.

High temp needed so KE is large enough to approach each other and overcome electrostatic repulsion.

High pressure needed to ensure that enough nuclei are close to each other for collision.

Nuclear fusion is the source of energy for a star. Without it stars collapse under their own weight.












left side = neutron rich and Beta- decay

right side = proton rich and Beta+ decay or due to electron capture

far right side or heaviest nuclides = alpha decay








The electronvolt ... 1 eV is the energy gained by an electron moving through an potential difference of one volt.

1 eV = 1.6 x 10-19 J