Bond polarity

A covalent bond among two atoms of similar element is explained as homopolar, one between different elements like heteropolar; the usual term bond polarity explains the unequal sharing of electrons among two atoms, and is a characteristic of heteropolar bonds when the two elements concerned have a distinct electronegativity. The more electronegative atom describes electrons and so acquires a partial negative charge, with the other atom becoming respectively positive. One manifestation of such type of polarity is the formation of an electric dipole moment, the magnitude of which is equivalent to the result of the charges and their average separation. In a series of molecules the dipole moments decrease like HF> HCl>HBr>HI as might be supposed from the falling variation in electronegativities.

Though, Dipole moments are, not all the time simple to interpret, as they can be affected by other issues, like the relative orientation of bonds in polyatomic molecules and the distribution of nonbonding electrons. Dipole moments are a significant source of intermolecular forces.

Polar covalent bonds can be considered as having some degree of ionic character, and the difference between 'covalent' and 'ionic' bond types is sometimes difficult to make. Some compounds have clear instances of both types of bonding concurrently. So CaCO₃ has well-described carbonate ions with C and O covalently bonded together; the complex ion also interacts ionically with Ca²⁺. Such type of complex ions requires not be discrete entities but can create polymeric covalent networks with a net charge, with the ionic bonds to cations (example silicates;). Even when only two elements are exists, though, bonding may be difficult to explain in simple terms. Under general conditions, when a compound is molecular it is usual to regard it as covalent (even though 'ionic molecules' like NaCl(g) can be made by vaporizing the solid compounds at high temperatures). When two elements of dissimilar electronegativity form a solid compound alternative explanations may be possible. Refer the compounds BN and BeO. Both form structures in which every atom is surrounded tetrahedrally by four of the other type (BN also has a substitute structure identical to that of graphite). This is a plausible structure on ionic grounds For BeO, given that the Be²⁺ ion must be much smaller than O^2-. Alternatively, many of the

Properties and structures of beryllium compounds are suggestive of some degree of covalent bonding. So, one can think of BeO as predominantly ionic but with the oxide ion polarized by the very small Be²⁺ ion so that electron transfer and ionic character are not complete. For BN the electronegativity variation among the elements is much less and it would be more natural to think of polar covalent bonding. The BN's tetrahedral structure can be understood from its similarity to diamond, where every carbon atom is covalently bonded to four others. The variation between two descriptions 'polar covalent' and 'polarized ionic' is not absolute but only one of degree. Which initial point is better cannot be laid down by rigid rules but is partly a matter of convenience.

One should be careful about the using oversimplified criteria of bond type based on physical properties. It is occasionally stated that 'typical' ionic compounds have high melting points and dissolve well in polar solvents like water, where covalent compounds have low melting points and dissolve well in nonpolar solvents. That can be very misleading.

Diamond is a purely covalent substance, has one of the highest melting points known and is insoluble in any solvent. Some compounds well explained by the ionic model have fairly low melting points; others are very insoluble in water on grounds that can be described perfectly satisfactorily in terms of ions.