  The way atoms attach to each other defines the function(s), e.g., mechanical,
optical, electronic, of a given material. The nature of the chemical bond is,
therefore, one of the most fundamental issues in materials. Both ionic
interactions, i.e., resulting from electrical charges associated with the
atoms, and covalent ones, i.e., the sharing of electrons between nuclei of
different atoms, are usually viewed as forces that attract between atoms to
form a rigid structure. Although less common for solid materials, it was shown
theoretically to be possible for covalent interactions at the chemically-active
electronic shell (or valence-band maximum) of semiconductors to reverse their
more common nature and become repulsive, i.e., act against bonding. Some
semiconductors with such predicted anti-bonding valence-band maximum levels
(such as halide perovskites) show experimentally some amazing (opto-)
electronic properties. Predictions that anti-bonding character can allow
tolerance for existing defects, at least in part, can explain the superior
properties of such semiconductors. Although there are known experimental ways
to estimate the degree of the covalent nature (e.g., electronegativity), this
was not possible hitherto for the type, i.e., distinguishing whether a material
exhibits bonding or anti-bonding covalent interactions. We have developed a
simple way to reveal the complete nature (both type and degree) of chemical
bonds, using experimental data. After confirming our development with classical
models and theoretical predictions, with a set of ~40 different functional
semi-conductors, we show how knowledge of the complete nature of covalent
bonding is of critical importance for fundamental properties of semiconductors.
