  Bulk tellurium (Te) is composed of one-dimensional (1D) helical chains which
have been considered to be coupled by van der Waals (vdW) interactions.
However, based on first-principles density-functional theory calculations, we
here propose a different bonding nature between neighboring chains: i.e., the
helical chains made of normal covalent bonds are connected together by
coordinate covalent bonds. It is revealed that the lone pairs of electrons of
Te atom participate in forming coordinate covalent bonds between neighboring
chains. Therefore, each Te atom behaves as both electron donor to neighboring
chains and electron acceptor from neighboring chains. This ligand-metal-like
bonding nature in bulk Te results in the same order of bulk moduli along the
directions parallel and perpendicular to the chains, contrasting with the large
anisotropy of bulk moduli in vdW crystals. We further find that the electron
effective masses parallel and perpendicular to the chains are almost the same
each other, consistent with the observed nearly isotropic electrical
resistivity. It is thus demonstrated that the normal/coordinate covalent bonds
parallel/perpendicular to the chains in bulk Te lead to a minor anisotropy in
structural and transport properties, distinct from a strong anisotropy observed
in the typical two-dimensional (2D) vdW materials such as graphite and MoS$_2$.
