  Hydrogen has been almost completely ignored in studies of the abundance
patterns of the chemical elements in solar energetic particles (SEPs). We seek
to find impulsive events where H fits these abundance patterns and document the
events that do not, suggesting possible reasons for the disparity. For 24 % of
the smaller impulsive SEP events, the relative abundance of H fits within one
standard deviation of the power-law fit of the abundances of elements 6 <= Z <=
56, relative to coronal abundances. In impulsive events with high intensities,
H can be 10 to 100 times its expected value. In a few of these larger events,
increased scattering at high wavenumber may preferentially detain H, perhaps
with self-amplified waves; in some events pre-event proton background may
con-tribute. In most large impulsive SEP events, however, associated shock
waves must play a much greater role than previously thought; fast (>500km/s)
coronal mass ejections con-tribute to 62 % of impulsive events. Shocks may
sample protons from the ambient coronal plasma or residual background as well
as reaccelerating heavier impulsive SEP ions injected from the region of
magnetic reconnection in solar jets. Excess H may be a signature of shock
acceleration.
