  The influence of spin-orbit interaction on chemical bonds in elemental solids
and homonuclear dimers is analyzed by means of density-functional-theory
calculations. Employing highly precise all-electron full-potential methodology,
our results represent benchmark quality. Comparison of the scalar- and
fully-relativistic approaches for elemental solids shows that the spin-orbit
interaction may contract or expand the volume of the considered material. The
largest variation of the volume is obtained for Au, Tl, I, Bi, Po and Hg,
exhibiting changes between 1.0--7.6\%. Using the tight-binding model, we show
for diatomic molecules that the nature of this effect lies in the angular
rearrangement of bonding and antibonding orbitals introduced by spin-orbit
coupling. Such an angular rearrangement appears in partially filled $p$- or
$d$-orbitals in heavy elements. Finally, we discuss the impact of the
relativistic effects on the chemical bonding in single-layer iodides and
transition metal dichalcogenides
