<Li> High melting point </Li> <P> Their strength, stiffness, and high melting points are consequences of the strength and stiffness of the covalent bonds that hold them together . They are also characteristically brittle because the directional nature of covalent bonds strongly resists the shearing motions associated with plastic flow, and are, in effect, broken when shear occurs . This property results in brittleness for reasons studied in the field of fracture mechanics . Network covalent solids vary from insulating to semiconducting in their behavior, depending on the band gap of the material . </P> <P> A standard ionic solid consists of atoms held together by ionic bonds, that is by the electrostatic attraction of opposite charges (the result of transferring electrons from atoms with lower electronegativity to atoms with higher electronegativity). Among the ionic solids are compounds formed by alkali and alkaline earth metals in combination with halogens; a classic example is table salt, sodium chloride . </P> <P> Ionic solids are typically of intermediate strength and extremely brittle . Melting points are typically moderately high, but some combinations of molecular cations and anions yield an ionic liquid with a freezing point below room temperature . Vapour pressures in all instances are extraordinarily low; this is a consequence of the large energy required to move a bare charge (or charge pair) from an ionic medium into free space . </P>

What force exists between atoms of metals that yield properties similar to ionic compounds