  The first eight elements of the periodic table are discussed: H, He, Li, Be,
B, C, N, and O. They are referred to as key elements, given their important
role in stellar evolution. It is noteworthy that all of them were initially
synthesized in the Big Bang. The primordial abundances of these elements
calculated using the Standard Model of the Big Bang (SMBB) are presented in
this review. The good agreement between the SMBB and observations of the
primordial abundances of the isotopes of hydrogen and helium, D, 3He, and 4He,
is noted, but there is a difference of ~0.5 dex for lithium (the isotope 7Li)
between the SMBB and observations of old stars in the galactic halo that has
not yet been explained. The abundances of light elements in stellar atmospheres
depends on the initial rotation velocity, so the typical rotation velocities of
young Main Sequence (MS) stars are examined. Since the data on the adundances
of light elements in stars are very extensive, the main emphasis in this review
is on several unsolved problems.
