PhET Alpha Decay Simulation
PhET Beta Decay Simulation
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Group Activity: Alpha
and Beta Radiation


See also Positron Emission and Electron Capture and
Mass Defect, Fission and Fusion

Radiation is an important natural phenomenon. Historically, radiation was the key to the first hints about the nature of the atom’s internal structure. Some atomic nuclei are unstable. That means that they emit particles and/or high-energy light spontaneously. All elements of atomic number 84 and higher are radioactive. Most other elements have radioactive isotopes.

You have been told that atoms do not gain or lose protons or neutrons. This is true if only chemical reactions are considered. There are two main types of nuclear reactions in which atoms gain or lose protons and neutrons. First, there is natural radioactive decay. In radioactive decay a heavy atom emits a particle or breaks up into 2 pieces without outside interference. Second, there is nuclear transmutation. Nuclear transmutation occurs when nuclei are bombarded with protons, neutrons or other atomic nuclei. In this process people can (and have) converted one element into another.

In this activity you will learn about different types of nuclear reactions and how to write and balance equations describing them.

Radioactive Particles/Processes
Type Symbol(s) Description
Alpha α,4  2α, 4  2He 2+ this is a type of atomic fission (when atoms split into two parts); it is really just the nucleus of a helium atom without any electrons; it results in the loss of two protons and two neutrons from the nucleus
Beta (electron) 0 -1β, 0 -1e- this is the emission of an ordinary electron from the nucleus; it results in the conversion of a neutron to a proton; it happens in nuclides with too many neutrons—when n0/p+ is too high
Beta (positron) 0+1β, 0+1e+ this is the emission a particle like an electron but with a positive charge: a positron; it results in the the conversion of a proton into a neutron; it happens in nuclides with too many protons— when n0/p+ is too low
Gamma γ,0  0γ this is the emission of a photon (a particle of light) with a very high energy; it has no effect on the nature of the nucleus and is simply the release of energy; other kinds of radioactive decay sometimes also include gamma rays
Neutron n0,1  0n neutrons can be emitted as part of a nuclear fission process in which a single nucleus splits into smaller nuclei
Proton 1  1p, 1  1H emitted protons are not natural radiation; they result from nuclear transmutation
Electron Capture 0 -1e- this process occurs when an electron is absorbed by the nucleus; the electron so ‘captured’ combines with a proton to produce a neutron

Chemical vs. Nuclear Reactions

Chemical Reactions
  1. Atoms are shuffled around and chemical bonds btw. atoms are broken and formed.
  2. Only electrons are involved.
  3. Only small amounts of energy are involved.
  4. Reactions can be influenced by things like pressure and temperature.
Nuclear Reactions
  1. Elements and isotopes change identity.
  2. In addition to electrons, protons, neutrons and other particles may be involved.
  3. Tremendous energy is involved.
  4. Not affected by temperature or pressure.
The above paraphrased from Chemistry, Chang, 7th edition. McGraw Hill, 2002.

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Alpha Decay

In Alpha Decay the nucleus of an atom splits into two parts. One of these parts (the α particle) flies away. The nucleus that is left behind is has its atomic number reduced by two and its mass number reduced by four. This is because an α particle is a helium nucleus consisting of two protons and two neutrons. Here is an example of an alpha decay process:

22288Ra  —> 4  2He 2+  + 21886Rn

The atom on the left side of the equation is the one that splits into two pieces. In alpha decay one of the two atoms on the right is always an α particle. Here is the key to getting these equations right: The other atom on the right always goes down by two in the atomic number and four in the mass number. Find the element that has the atomic number you calculate and use its symbol. Here is another example:

20884Po  —> 4  2He 2+  + 20482Pb

Rules for Balancing Nuclear Equations
For the following nuclides, write the equation for alpha decay.

  1. 256103Lr
  2. 23191Pa
  3. 22589Ac
  4. 21187Fr
  5. 18579Au
  6. 14662Sm
  1. 23392U
  2. 14964Gd
  3. 23290Th
  4. 17578Pt
  5. 23793Np
  6. 23490Th

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Beta Decay

In Beta Decay a neutron in the nucleus of an atom breaks down (decays). When it does this, it becomes a proton. It emits an electron and an anti-neutrino. (Don’t worry about what the anti-neutrino is doing here.) The nucleus that is left shows an increase of one in the atomic number. The atomic mass number is unchanged. Here is an example of an beta decay process:

14 6C  —> 14  7N  + 0-1β

The nuclide that decays is the one on the left-hand side of the equation. Notice that the resulting nucleus has an atomic number one greater than the decaying nucleus. The -1 (β) and 7 (N) add up to the 6 (C). Here is another example:

13153I  —> 13154Xe  + 0-1β

For the following nuclides, write the equation for beta decay.

  1. 6  2He
  2. 20179Au
  3. 5226Fe
  4. 4219K
  5. 9038Sr
  6. 23993Np
  1. 24 11Na
  2. 24795Am
  3. 8235Br
  4. 9943Tc
  5. 8  3Li
  6. 3215P
Identify the type of decay and complete the nuclear equation.
  1. 13  5B  —> 13  6C + _____
  1. _____   —> 4  2He 2+  + 18676Os
Last updated: Jan 06, 2008              Home