Lack of stability after radiation

  1. U = Th + alpha rays
    92p 90p
    146n 144n

    I have heard that if n/p ratio exceeds 1.56, the substance becomes radioactive. Now, for Uranium, n/p ratio is 1.59. It gives out alpha rays in order to gain stability and in turn forms Thorium. n/p of thorium is 1.6. The ratio increases. This means Thorium is more unstable than Uranium. Then why on earth does Uranium give off alpha rays? It only leads to more instability.

    Thanx in advance...
  2. jcsd
  3. SteamKing

    SteamKing 8,374
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    You are assuming the process stops when U turns into Th. It does not. Th-234 is itself radioactive with a very short half-life of about 24 days. The decay process continues, producing various other radioactive elements, until the final element, lead, is reached. Lead-206 is the end product of the decay of U-238.

    For more information, see the Radium-series (or Uranium-series) for the complete decay chain in the following article:
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  4. Thanx you @SteamKing. My concept is clear now. But one silly question I would like to ask you, why are alpha rays given out at all when it leads to more instability? I mean, if, in a nucleus, n decreases by 2 and p decreases by 2, n/p ratio will always increase. Therefore it makes no sense to give off alpha rays. Some other rays would have been better ;P

    Thanx again...
  5. SteamKing

    SteamKing 8,374
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    There are three common types of radioactive decay: alpha emission, beta emission, and gamma emission.

    Gamma emission does not alter the number of protons or neutrons in the nucleus, so the chemical properties of the element are unaffected.

    Alpha emission reduces the number of protons by 2 and the atomic weight by 4, therefore the element undergoing decay changes chemical properties.

    Beta emission increases the number of protons by 1 and reduces the number of neutrons by 1, thus leaving the atomic weight unchanged. The element changes chemical properties also due to this radiation.

    Like I said earlier, radioactive elements go through a series of different changes until they reach atomic stability. It can't be done (usually) in one fell swoop.

    For more on radioactive decay:
  6. Thanx again...
    I understand what you say. But I still wonder why uranium gives off alpha rays. I mean, these alpha rays will only lead to more instability. So why not give off beta rays nstead...? What makes it so necessary for uranium to give off alpha ray? Would not beta rays be better? It would have at least reduced the ratio, isn't it?

  7. SteamKing

    SteamKing 8,374
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    Like I explained, if U-238 gave off beta rays, the atomic number goes up by 1, so U-238 would become Neptunium-238 (half-life about 2 days). A further hypothetical beta emission would produce plutonium-238 (half-life about 88 years).

    The naturally occurring elements all have atomic numbers of 92 and below. As of the present, no stable elements with atomic numbers greater than 92 are known to exist. Some isotopes of elements with atomic numbers > 92 have relatively long half-lives, but eventually all undergo radioactive decay such that the atomic number declines and they wind up as lighter, more stable elements. There are several other decay series (like the radium series) which lay out this decay progression.

    As to why a given nucleus prefers a certain type of emission, that's beyond my level of knowledge.
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  8. @SteamKing, thanx a lot...!
    This was exactly what I needed.
    Thank you everyone at PF...
    Thanx again....
  9. mfb

    Staff: Mentor

    Atoms have no "plan" like "I have to go to stable isotopes". If a decay is possible in terms of energy, it will happen after a while. If multiple decays are possible (usually alpha and beta for some heavy nuclei, sometimes fission), they will all occur with some relative frequencies.
    The decay rates for beta decays are tricky to evaluate, for alpha decays there is a strong correlation between decay energy and lifetime. The more energy the decay releases, the shorter the lifetime (with some exceptions).
  10. It is true that decay chains exists, but your argument is backwards and wrong. Uranium does not decay into thorium because the chain exists, instead the decay chain exists because uranium is unstable and it decays into thorium which is also unstable and decays into another unstable isotope. The series of decay will continue until you reach a stable isotope.

    There are basically 3 reasons why atoms decay. The neutron to proton ration is too high for stability (they have too many neutrons), the neutron to proton ration is too low (they have too few neutrons), or they have too many nucleons in general (the nucleus it too big).

    The mode of decay is determined by the cause of instability. Nuclei that have too many neutron typically under go beta minus decay. Here a neutron decays into an electron and a proton (the is also an electron anti-neutrino). This decrease the neutron to proton ration. Nuclei that have too few neutron typically under go beta plus decay. Here a proton decays into an positron and a neutron (there is also an electron neutrino) increasing the neutron to proton ratio. Finally large nuclei decay by alpha decay which decreases their size.

    Uranium enters this third case. Its nucleus is too big for stability. Thus it will undergo alpha decay.
  11. One possible reason of radiation of alpha rays, as many people are saying, is that the He++ ion thus released is very stable. Thus, at least one stable compund is formed out of the unstable radioactive element.

  12. Astronuc

    Staff: Mentor

    The alpha particle is tightly bound, certainly, but it is not the reason that it is released. Until Pb or Bi are reached, the much larger nucleus following alpha decay also decays, some by alpha, some by beta decay, and some by either.

    For some heavier transuranic nuclei, spontaneous fission is also a possibility, and most fission products decay in a decay chain, some including neutron emission, which is the source of delayed neutrons in nuclear reactors.
  13. Yes, but it is very rare for any nucleus to release the also tightly bound nuclei like carbon 12 or oxygen 16.
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