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Positrons, electrons, and chemical reactions

  1. May 14, 2014 #1
    Hello all,

    In chemistry class we recently began the subject of nuclear chemistry. I'm sure you all know that nuclear chemistry unleashes a swarm of new particles. My teacher mentioned the Positron, a particle few of my classmates recognized. To help us understand its nature we were told to think of it is a positive electron.

    Why has it taken a whole year of Chemistry class to learn about the electron's brother? Why don't positrons affect chemical reactions at all? Are they simply less frequently seen than electrons?

    Furthermore, what IS a positron? I've heard of physicists describing it as an electron going backwards in time... which is completely befuddling.

    Any insight would be appreciated.
  2. jcsd
  3. May 14, 2014 #2


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    A positron can be thought of as the anti-matter counterpart of the electron:


    Positrons don't have much effect on chemical reactions because they simply don't exist in large quantities in the natural environment on earth. Positrons are useful, however, in things like PET scans for medical diagnostics, but the positrons used in those applications must be manufactured, just like X-rays are, using a special machine.

    That you haven't received any exposure to the existence of positrons in you chemistry courses shouldn't reflect on the skill or knowledge of your instructors, however. For chemical reactions, most of the action takes place in the electron orbitals surrounding the various atoms involved.

    As a science, chemistry evolved quite happily for several hundred years without having detailed knowledge of the atomic nucleus. Like the positron, the neutron was only theorized to exist in the 1920s and then discovered in 1932 by James Chadwick.


    The neutron, however, became very useful in advancing atomic physics, particularly in research on artificial radioactivity and then the development of atomic energy. Outside of knowing how many neutrons a particular nucleus contains so that you can compute the atomic weight of an atom, I'll bet your chemistry work hasn't said much about neutrons, either.
  4. May 15, 2014 #3


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    I don't want to open Pandora's box directing you to Dirac's theory of the positron, so I want to give you an analogy from solid state physics which is easier to grasp for chemists than relativistic quantum mechanics.
    In an isolator, you have the valence and conduction band, the valence band being completely filled with electrons and the conduction band being empty. Usually, you won't notice the charge of the electrons in the valence band as it's compensated by the positive charge of the atomic nuclei making up the solid.
    Now, by absorption of a photon (a quantum of light) you can excite an electron from the valence to the conduction band. It will leave behind a hole which is positively charged, as the nuclear charge is now uncompensated.
    This hole is the analog of the positron in particle physics. Instead of saying you excited an electron you can say you created an electron-hole pair. or, you created an electron in the conduction band and anihilated an electron in the valence band. The latter statement can also be formulated as: you created an electron in the conduction band and created a hole in the valence band. Apparently, creation of a hole in the valence band and anihilation of an electron in the valence band are equivalent statements. But creation is the time reversed process of anihilation and vice versa. So you see that formally, a hole (or positron) can be described as an electron which moves backward in time.
  5. May 15, 2014 #4
    Positrons don't exist in large numbers on Earth. (they are created as the result of radioactive decay as well as the P-P fusion in the Sun). On Earth, they are almost immediately annihilated or absorbed. They don't participate in any significant chemical reactions, they're not around long enough. All sub-atomic particles either have an antiparticle OR are their own antiparticle. An antiparticle can be thought of as being identical to its particle, except of opposite charge, as well as causing the annihilation of the particle when the two collide. For some calculations, it is conveniant to think of them as electrons going backward in time. This isn't chemistry, its physics and in physics the Laws of Physics are the same forward or backward (almost exactly). Once you consider space-time as being 4 dimensional, it is sometimes easier to just have one type of particle which can move in any direction (including forward or backward on the time axis) than to need to keep track of two types. I'd suggest you not worry about that, until you take the upper level physics courses where you can decide for yourself how "real" traveling backwards in time for these particles is. My take is that it works for some purposes, so isn't wrong, but it isn't actually right, either. Oh, did you know that neutrons, if not contained in an atomic nucleus, will decay with a half-life of about 15 minutes?
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