  Relativistic and QED corrections were calculated for hyperfine splitting of
the $2S_{1/2}$ ground state in $^{6,7}$Li atoms with an exact account for
electronic correlations. The resulting theoretical predictions achieved such a
precision level that, by comparison with experimental values, they enable
determination of the nuclear properties. In particular, the obtained results
show that the $^7$Li nucleus, having a charge radius smaller than $^6$Li, has
about a 40% larger Zemach radius. Together with known differences in the
electric quadrupole and magnetic dipole moments, this calls for a deeper
understanding of the Li nuclear structure.
