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The minimum energy which must be supplied to extract an electron from a solid; symbol , units J(joule), or more often eV (elecronvolt). It is a measure of how tightly electrons are bound to a material.
http://invsee.asu.edu/nmodules/engmod/propwf.html

Protons are observed to be stable and their theoretical minimum half-life is 1×1036 years. Grand unified theories generally predict that proton decay should take place, although experiments so far have only resulted in a lower limit of 1035 years for the proton's lifetime. In other words, proton decay has never been witnessed and the experimental lower bound on the mean proton lifetime (2.1×1029 years) is put by the Sudbury Neutrino Observatory[3].
However, protons are known to transform into neutrons through the process of electron capture. This process does not occur spontaneously but only when energy is supplied. The equation is:
P+ + e- n + νe
where
p is a proton,
e is an electron,
n is a neutron, and
νe is an electron neutrino
The process is reversible: neutrons can convert back to protons through beta decay, a common form of radioactive decay. In fact, a free neutron decays this way with a mean lifetime of about 15 minutes.
http://en.wikipedia.org/wiki/Proton