  The relative abundances of chemical elements and isotopes have been our most
effective tool in identifying and understanding energetic particles. The early
surprise in solar energetic particles (SEPs) was 1000-fold enhancements in
3He/4He from resonant wave-particle interactions in the small "impulsive" SEP
events that emit electron beams that produce type III radio bursts. Further
studies found enhancements in Fe/O, then extreme enhancements in abundances
that increase with mass-to-charge ratio A/Q by a factor of 1000 from He to Pb
arising in magnetic reconnection on open field lines in solar jets. In
contrast, in the largest SEP events, the "gradual" events, acceleration occurs
at shock waves driven out from the Sun by fast, wide coronal mass ejections
(CMEs). Averaging many events provides a measure of solar coronal abundances,
but A/Q-dependent scattering during transport causes variations with time; thus
if Fe scatters less than O, Fe/O is enhanced early and depleted later. To
complicate matters, shock waves often reaccelerate impulsive suprathermal ions
above active regions that have spawned many impulsive events. Since both
impulsive and gradual SEP events have abundance enhancements that vary as
powers of A/Q, we can use abundances to deduce Q-values and 1 - 3 MK source
plasma temperatures during acceleration. Comparing coronal abundances from SEPs
and from the slow solar wind as a function of the first ionization potential
(FIP) of the elements, remaining differences are for the elements C, P, and S.
The theory of the fractionation of ions by Alfv\'en waves shows that C, P, and
S are suppressed because of wave resonances during chromospheric transport on
closed magnetic loops but not on open magnetic fields that supply the solar
wind. Shock waves can accelerate ions from closed coronal loops that easily
escape as SEPs, while the solar wind must emerge on open fields.
