Read
Read pages 790 and 791 in the text. Also, review "Table 16.1" on page 792.

Nuclear Decay

There is one question about the nucleus that has yet to be addressed.  If the nucleus is a ball of positively charged protons, why don't these like charges fly apart?  Recall from previous units that Coulomb's law of electrostatic forces would indicate that the positive protons should repel each other, leading to a breakdown of the nucleus.  In fact, some atoms do break down because of this electrostatic force.  This is called nuclear decay .  What, however, stops the atom from totally breaking apart from repelling protons?


Nuclear Forces


The nucleus is held together by what physicists call the strong nuclear force.  This fundamental force of nature counters the electrostatic force of repulsion that would exist between the protons in the nucleus.  For example, the gravitational attraction between two protons 5 cm apart is 7 × 10-36 N, while the electrostatic force of repulsion at the same distance is 9 N.  Just making up the difference between these two fundamental forces would require a nuclear force that is 1037 times stronger than gravity.  The strong nuclear force is massive compared to the gravitational force; but the gravitational force extends throughout the universe, whereas the strong nuclear force can act only over distances that are - relative to the size of the nucleus (~10-14 m) - extremely small.

So, why are some atoms stable and others unstable?  And what is the purpose of all these neutrons?  The strong nuclear force is almost independent of electric charge but acts only over a very short distance.  The strong force exists between any nucleon pair, whether it's a proton-proton, neutron-neutron, or proton-neutron.  Even though the strong nuclear force is powerful, the electrostatic repulsive force has a longer range of action.  This means that one proton is repelled by every other proton in the nucleus but is attracted only to its nearest neighbours.



The four fundamental forces are as follows:

  1. gravitational force

  2. electromagnetic force

  3. weak nuclear force

  4. strong nuclear force

Force

Relative Strength

Range

strong nuclear

1

≈10-14 m (nucleus)

electromagnetic

0.0073

∞

weak nuclear

10-9

≈10-18 m (nucleon)

gravitational

10-38

∞

As protons are added to a nucleus (moving down in the periodic table to heavier elements), more neutrons need to be added to balance out the additional repulsive electrostatic forces.  At some point, however, adding neutrons no longer helps. All nuclei with more than 83 protons are unstable and will decay spontaneously, like americium-241 in the smoke detector, which has 95 protons.


Try This
Complete "Practice Problems" 1 and 2 on page 792 of the textbook.

 

Scientists working with radioactive substances discovered that helium gas was invariably present in their experiments.  Rutherford proposed that helium gas was produced by the radioactive substances and that the alpha particle, known to be produced by radioactive materials, was simply a helium nucleus missing its electrons.  When Rutherford experimented with radon, he found that radon spontaneously split into an alpha particle (a helium nucleus) and a polonium atom.  This natural change from one element to another is called transmutation.  The original element is known as the parent element and the new element is called the daughter element.



Transmutation: decay or change into a different element



Parent element: the original element in a decay process



Daughter element : the element produced by a decay process


Watch This
Watch the short video about the decay of carbon - 14  Radioactive Decay of Carbon-14


Because the transmutation of radon to polonium produced an alpha particle, it is called alpha decay.  If the particle emitted is a beta particle, it is called beta decay.  The term decay refers to a larger particle splitting into smaller particles.