  We study the production of very light elements ($Z < 20$) in the dynamical
and spiral-wave wind ejecta of binary neutron star mergers by combining
detailed nucleosynthesis calculations with the outcome of numerical relativity
merger simulations. All our models are targeted to GW170817 and include
neutrino radiation. We explore different finite-temperature, composition
dependent nuclear equations of state and binary mass ratios, and find that
hydrogen and helium are the most abundant light elements. For both elements,
the decay of free neutrons is the driving nuclear reaction. In particular,
$\sim 0.5-2 \times 10^{-6} M_{\odot}$ of hydrogen are produced in the fast
expanding tail of the dynamical ejecta, while $\sim 1.5-11 \times 10^{-6}
M_{\odot}$ of Helium are synthesized in the bulk of the dynamical ejecta,
usually in association with heavy r-process elements. By computing synthetic
spectra, we find that the possibility of detecting hydrogen and helium features
in kilonova spectra is very unlikely for fiducial masses and luminosities, even
when including non local thermodynamics equilibrium effects. The latter could
be crucial to observe He lines a few days after merger for faint kilonovae or
for luminous kilonovae ejecting large masses of helium. Finally, we compute the
amount of strontium synthesized in the dynamical and spiral-wave wind ejecta,
and find that it is consistent with (or even larger than, in the case of a long
lived remnant) the one required to explain early spectral features in the
kilonova of GW170817.
