The Subatomic Realm: Dramatis Personae

Let's meet the cast of characters who populate the world of the atoms. You may not see some of these very often in the Modern Physics class, but they will pop up if you continue to study physics, or even read the newspapers.

The Big Three: proton, electron, neutron

These three particles are often described as the "building blocks of matter." One can put them together to form any element. Although we have discovered that the proton and neutron may be composed of even smaller sub-units (quarks), it is still useful and convenient to treat them as fundamental units. The electron, as far as we can tell, is truly fundamental: it can't be broken down into any smaller pieces.

The electron and proton have opposite electric charges, and so are usually found in equal numbers. The neutron has no electric charge (hence its name). All atoms contain electrons and protons, and all but one (ordinary hydrogen) contain neutrons as well. The "light" elements usually have equal numbers of protons and neutrons, but neutrons outnumber protons in the heavier elements. One isotope of gold, for example, has 118 neutrons but only 79 protons.

Your textbook's Appendix of Atomic Data provides information on the different isotopes of all the elements. If you read it carefully, you can figure out exactly how many protons and how many neutrons are in a particular isotope. It has additional information, too: the half-life of radioactive isotopes, and the relative abundance of stable isotopes.



  Q:  How many protons and neutrons are
      in Copper-66?

  Q:  How many neutrons are in the most
      common form of Hafnium?

  Q:  What is the longest-lived isotope
      of Technetium?

Peek at the answers.

The electron and proton are both stable particles. The neutron, on the other hand, is unstable in isolation: it will decay into a proton and electron (and something else -- see below) with a half-life of about 886 seconds. If it is surrounded by other neutrons and protons, however, the neutron will not decay.

Antiparticles

All particles have corresponding antiparticles, which have the same mass and (if applicable) size, but opposite charge. The electron's partner, the antielectron, is so common that it has a name of its own: the positron.

When a particle meets its antiparticle, the two annihilate each other, turning their combined rest mass into energy according to Einstein's equation


                       2 
         E   =    m * c

It's obvious that the positron isn't an electron, because its charge is positive, not negative. In a similar way, an antiproton is easy to distinguish from a regular proton. But how can one tell an antineutron from an ordinary one? Their electric charge is the same: zero.

In diagrams and equations, physicists often use a horizontal bar placed directly over the symbol for a particle to mean "this is the anti- version".

The Radiation Squad

Near the end of the nineteenth century, scientists investigated the phenomenon of radioactivity. Some materials gave off heat and energy as they transmogrified into other materials. For example, radium emits radiation as it turns into radon, which in turn emits radiation as it becomes polonium, and so on down a chain of seven more reactions until it reaches a stable isotope of lead. It eventually became clear that there were three classes of radiation.

alpha radiation does not penetrate matter well; a few inches of air or a single sheet of paper will stop it. Later research showed this radiation to be little bullets made of two protons and two neutrons, a.k.a. "alpha particles." If one adds two electrons to an alpha particle, one makes an atom of helium.
beta rays penetrate matter more efficiently than alpha rays: they can travel several meters in air or several centimeters into the body. They turned out to be electrons moving at high speed.
gamma rays are very penetrating: they can travel hundreds of meters through air and easily reach any part of the body. They turned out to be true "rays": very energetic photons, with wavelengths far below a nanometer.



                    mass (kg)          charge (C)
------------------------------------------------------------
  alpha                         

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  beta

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  gamma

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For more information

Learn basic properties of radiation from this page from the Princeton University Environment Health and Safety program.