How do we rationalize the covalent bond? And how is metallic and ionic bonding conceived?

1 Answer

Traditionally, we regard the covalent bond as the sharing of electron density between atoms......


The modern covalent bond is regarded as a region of high electron density formed between two positively charged atomic nuclei such that internuclear repulsion is negated and a net attractive force between the nuclei to the electron cloud results.......This is the reason why we invoke electron sharing for a covalent bond......

#H* + *HrarrH-H#

i.e. where #*H# is conceived to be a hydrogen radical, an atomic hydrogen nucleus with one unpaired electron. The two electrons form a covalent bond.

On the other hand, ionic bonding is commonly rationalized by the transfer of electrons between species, typically from a metal TO a non-metal such that discrete positive and negative ions are generated, and electrostatic forces of attraction bind the ions together in an infinite, non-molecular array in which attraction between ions of UNLIKE charge overcome the electrostatic repulsion between ions of LIKE charge in the matrix, and a net attractive force results.

e.g. #Na+1/2Cl_2rarrNa^(+)Cl^(-)#

Metallic bonding is also characterized by non-molecular interaction, in which a close-packed array of metal atoms donate 1 or 2 or more electrons to the overall lattice. Metallic bonding is commonly described as #"positive ions in a sea of electrons"#. Because the metal ions can move with respect to each other without weakening the metallic bond, metals are typically both #"malleable"#, capable of being hammered out into a sheet, and #"ductile"#, capable of being drawn out into a wire.

The so-called delocalization of electrons invoked for metallic bonding can also rationalize both the thermal and electrical conductivity of most metals, in that the free electrons act as carriers for heat and charge........

The distinction between molecular bonding (largely due to covalent bonding interaction), and non-molecular interaction (which characterizes ionic and metallic bonding) is clear.

Molecules can have differing degrees of intermolecular interaction, hydrogen bonding, where hydrogen is bound to a strongly electronegative atom, or dipole-dipole interactions if the molecule contains a dipole.