Module 8

Module 8—Nuclear Decay, Energy, and the Standard Model of the Atom

 

Try This

 

TR 2. A singly ionized carbon atom is accelerated by a parallel-plate apparatus and passes through a velocity selector with a magnetic field strength of 0.950 T and an electric field strength of 5.60 × 105 V/m. The ion then passes into a mass spectrograph with a magnetic field of 1.50 T and the sensor detects a radius of 4.89 × 10–2 m.

1. What is the velocity of the carbon ion as it passes through the velocity selector?

2. What is the mass of the carbon ion as determined by the mass spectrograph?

3. If the carbon ion has an atomic mass of 12, what is the value of 1 atomic mass unit?

Read

 

The preceding information can also be found in your physics textbook on pages 790 and 791. “Table 16.1” on page 792 lists the masses of subatomic particles in both kilograms (kg) and atomic mass units (u).

 

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 Module 3: Lesson 2 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.

So, while there is evidence of some nuclear decay, what 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. It 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.

 

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 the 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.

 

Watch and Listen

 

Watch an animation of the parent carbon-14 decay into the daughters nitrogen-14, electron, and electron antineutrino in this Natural Transmutations/Decay Animation.

 

Since 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.

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