how do you recognize alpha, beta and gamma radiation decay?

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For any particular nuclide, the way to tell how it decays is to look it up in a reference site.

Carbon-14, for example, decays to Nitrogen-14. Since the daughter product has the same atomic mass number, 14, any particle emitted has to carry no mass units. It has to be a lot lighter than a proton.
Since the atomic number increases, meaning the daughter product has one more unit of charge in the nucleus, any particle emitted has to carry a charge of -1.
The particle that qualifies is an electron, which is a beta particle.

Radium-226 decays to radon-222. Here, the mass number decreases by 4 and the atomic number decreases by 2 (from 88 to 86). Whatever is emitted has to have 4 units of mass and 2 units of charge. The particle in this case is a helium nucleus, or an alpha particle.

Carbon-11 decays to boron-11. Again, these are isobars (they have the same atomic mass number). In this case, the atomic number decreases by 1, so the particle emitted has to carry a mass of 0 and a charge of +1. Carbon-11 decays by positron emission.

There are very few cases of decay by pure gamma ray emission. These are generally emissions that follow some other decay. For example, molybdenum-99 decays to technetium-99 by beta decay, but the technetium-99 is in an excited state. It's called a metastable state since it takes a long time for the nucleus to rearrange itself and shed its excess energy in a gamma ray, so it's called technetium-99m.
Technetium-99m decays to technetium-99 by emitting a gamma ray. Since the atomic mass number stays the same, and the charge stays the same, any particle emitted must carry no mass units and no charge. A photon, or gamma ray, qualifies....Show more