|This week we have learned about alpha, beta, and
gamma nuclear radiation and decay. We know that
in beta decay a neutron is lost and a proton is
gained, and that the daughter element has one
more proton than its parent. For example, when
Thorium-234 beta decays into Protactinium-234.
This has aroused two questions: If the number of
electrons is the same as that of the protons in
normal atoms, does the new Pa-234 get a new
electron in the electron cloud, and if so, where
does it come from? Also what happens to the
electrons that are left over when radioactive
elements like Uranium-238 alpha decay? Does
alpha decay somehow include a loss of 2 electrons
so that the daughter element is not an ion?
|Question Date: 1999-02-05|
You have asked a very good question. It turns out
that in the field of particle/nuclear physics
certain assumptions have been made that as far as
we can tell are true. These assumptions are that
in reactions like you have mentioned above
certain quantities are conserved such as energy,
charge, momentum, and some other more confusing
quantities such as baryon and lepton
So, if you take the beta decay
reaction Th-234 -> Pa-234 where a neutron turns
into a proton, is charge conserved in this process
as is? If charge is not conserved then there must
be another particle involved in the process that
we are missing so that charge is conserved in the
reaction. What must that other particle
If you remember the idea that charge
must be conserved (and that you really only have
protons, electrons, and neutrons to work with) you
should be able to answer all your questions.
Remember that atoms find a way to have zero net
charge, if possible, due to the attraction between
protons and electrons.
As an aside:
protons and neutrons are part of a family called
baryons and the number of baryons is conserved in
any reaction. Electrons and neutrinos are called
leptons and their number is also conserved. If
you haven't heard of neutrinos yet you might want
to take a few minutes to find out about them.
They play a role in the reactions you asked
Beta decay, as you said, effectively consists on a
neutron decomposing into a proton and an electron
(the beta particle), which gets ejected at high
speed. Now, that beta particle is just as much an
electron as any other, and will roam about, lose
energy through collisions with other atoms, and
eventually be attracted to, and fall into orbit
about a positive ion.
The atom which
decayed, as you observe, is now positively
charged, and will attract electrons the same as
any other ion would. These electrons could come
from the "electron cloud" (in which case, the beta
particlehas also joined the cloud) or it's evn
possible (but perhaps not very likely) that the
escaped beta particle will end up orbiting the
same nucleus that created it!
look at the alpha-decay of U-238. After the decay,
we have Th-236, with two extra electrons, and
He-4, missing two electrons. Sometimes the outer
shell electrons can be knocked out of place by the
escaping He nucleus, but if they don't, we just
have two charged ions floating around (okay, one
of them at high speed!). Now you tell me,
Hope this helps...
I will make a stab at this -- even though Physics
has changed so much recently, an expert might need
to correct me. In beta decay, the effective
outcome is that a neutron decays into a proton, an
electron, and a neutrino. It is rare, however,
that the atom could retain this electron since it
carries a significant amount of the released
energy kinetically. i.e. It is traveling way too
fast for the atom to retain it. You re correct in
assuming that the resulting atom is an ion-- In
general, the atom will steal an electron from a
surrounding atom to which it is less strongly
bound. (This of course still leaves a charge
imbalance.) The ejected electron itself is likely
to scatter other electrons from nearby atoms,
creating even more ions. (This is why highly
energetic nuclear radiation is called "ionizing").
There are several more mundane mechanisms for
the charges to balance again, electrical
conduction, plasma motion due to the field etc.
Alpha emission is also capable of local
ionization, but typically over a shorter distance
because the alpha particle is more likely to
scatter and lose energy.
If the radioactive
sample is isolated (i.e. suspended in air for
example) it will create ions in a cloud
surrounding itself-- this effect has practical
application. You might check out how a common
smoke detector works-- There are also radioactive
tipped lightning rods. Why do you think this is a
By the way: there is a nice MPG
movie of beta decay
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