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Author: Andrew R. Barron

If we now understand that radioactive decay via the loss of an α particle results in the decrease of the atomic number by two. In contrast, thorium-234 decomposes to protactinium-234 by the loss of a β particle (high energy electron). Protactinium has an atomic number of 91, while thorium’s atomic number of 90. This observation initially caused consternation.

In the 1920’s people thought that the nucleus contained electrons, based on the concept that if a coin falls out of your pocket then your pocket must have contained a coin. But in the case of radioactive decay via the loss of a β particle this cannot be true since the nucleus only contains protons and neutrons. So, if there are no electrons in the nucleus where does the β particle come from?

If we consider the data in the following tabe, note that the mass of a neutron is greater than the mass of a proton plus the mass of an electron.

Particle Charge Mass (kg)
Proton +1 1.6726 x 10-27
Neutron 0 1.6749 x 10-27
Electron -1 0.00091 x 10-27

Therefore, we can consider the neutron to be comprised of a proton and an electron and energy:

Thus, a neutron may decompose to a high energy β particle

In this process there is no change in mass number, but there is an increase in one atomic number. This may be shown by the decomposition of thorium-234 to protactinium and a β particle.

Last Update: 2011-02-20