Is it possible to add a proton to an atom? What would happen?
Considering that the atom would then have one fewer electrons that
protons, would it not become an ion?
Yes, it is possible, and if nothing lse happened you would have produced a
positive ion. Usually, though, what you produce is an unstable nucleus that
has lost an electron or so.
First, the electrons: It's jarring for an electron to suddenly find itself
associated with a nucleus with more charge than the one they had settled
into. As they settle into new "orbits" some electrons might acquire enough
energy to break free of the nucleus.
Second, the nucleus: There's a war going on, in nuclei, between the attractive
short-range nuclear force that holds protons and neutrons together and the
repulsive electromagnetic force between protons. Nuclei with too many protons
are unstable and break into fragments.
Also, it takes a little extra energy to get a proton into the nucleus. Since
the repulsive electromagnetic force is longer range than the attractive
force, a proton almost but not quite touching the nucleus feels only the
force. You have to give the proton extra energy to shove it in, and then
settles, it has more energy than it needs. Somehow, the nucleus must get
this extra energy. One way is to spit out some other particle.
Tha hard part would be getting iot to happen. A proton would be
repelled by the atom's positively-charged nucleus. You would have to fire the
proton right at the nucleus very fast to get past this repulsive force. Once
it got close enough, the "strong force" that holds the nucleus together
(did you ever wonder what makes all those positively-charged protons clump
into a ball?) could hold on to it.
Once that were accomplished, you are right, the atom would be an ion,
because the number of electrons and protons would not be equal. The
nucleus itself might also be unstable. That of course depends on how many
protons and neutrons were in the nucleus before the proton was added.
Richard E. Barrans Jr., Ph.D.
PG Research Foundation, Darien, Illinois
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Update: June 2012