Ionic and covalent bonds Polar

Ionic and covalent bonds
a. Ionic bonds
We have seen above in our consideration of ionization energies and electron affinities that, in accord with Lewis’s insight, atoms have a strong tendency to attain noble gas electronic configurations. This can be achieved by complete electron transfer between atoms of different elements if their electronegativities are sufficiently different. 

The moreelectropositive atom donates one or two electrons to the more electronegative atom and the two become a cation and an anion, both with the noble gas electronic configuration. There is then a strong electrostatic attractive force between the ions which constitutes an ionic bond. The classic example is sodium chloride, NaCl.

Depending upon the particular compound and the solvent, the cations and anions of an ionic compound M+ X- may exist as ion pairs in solution, held in contact electrostatically, or M+ and X- may dissociate and diffuse through the solution independently. At extremely high temperatures in the gas phase, some very stable ionic compounds do exist as ion pairs.

b. Covalent bonds
Electron transfer between atoms of two elements whose electronegativities are not very different will not be energetically favourable (the cost of removing an electron from one will not be provided by the energy of adding an electron to the other and, on the molar scale, the lattice energy). However, if atoms of elements need to gain electrons (rather than lose them) in order to attain the noble gas configuration, they can achieve this by sharing electrons. In eqn 1.6, for example, two fluorine atoms, each with seven valence electrons, can both achieve the noble gas configuration if they share a pair of electrons. This sharing of a pair of electrons constitutes a covalent bond and the two shared electrons are called a shared (electron) pair or a bonding (electron) pair. Most covalent bonds are formed between atoms of different elements, e.g. as shown for HCl .

One atom may share two or more pairs of electrons by making bonds with two or more other atoms in order to bring its valence shell occupancy up to eight electrons. An oxygen atom has six valence electrons, so needs two more, and forms covalent bonds with two H atoms, eqn 1.8, for example; its valence shell then comprises two bonding pairs and two unshared pairs of electrons. Just as a shared electron pair in the valence shell of an atom in a molecule is called a bonding pair, an unshared electron pair is often called a nonbonding pair, or a lone pair.

A carbon atom (4 valence electrons) achieves a filled valence shell (an octet) by making four bonds, e.g. with H atoms (eqn 1.9) so its filled valence shell comprises four bonding pairs. The four simple examples collected together in Scheme 1.3 show circled atoms which have achieved complete valence shells (octets for all except H whose 1s shell is full with just two electrons) by sharing electron pairs; only C and H do not include lone pairs in their complete valence shells.

Polar covalent bonds and dipoles
When a covalent bond is formed between atoms of different electronegativity, the bonding pair of electrons is not shared equally but is attracted towards the more electronegative atom. Such a bond is said to be polarized, or described as a polar bond, and the charge separation is represented by symbols δ- and δ+ which represent partial negative and positive charges.

If the difference in electronegativity is very large, an electron would be completely transferred from the less to the more electronegative atom to give a cation and an anion, and the bond becomes ionic (as discussed above). Bonds between atoms of non-metallic elements (e.g. C, N, O, and halogens) are typically covalent, and bonds between atoms of elements more electronegative than carbon (N, O, and halogens) and electropositive elements (metals) are usually ionic. However, the borderline between ionic and polar covalent bonding is not clear.

Would a bond between each of the following pairs of atoms be covalent or ionic?

(a) O, H   

(b) C, F    

(c) Li, F 

(d) C, Mg

Solution
The bond is ionic only when the electronegativity difference is appreciable (typically, >1.8 on the Pauling scale); otherwise, the bond is covalent.

(a) covalent 

(b) covalent 

(c) ionic 

(d) covalent

The charge separation (polarity) of a covalent bond may also be represented as a dipole with an arrow pointing from the positive end to the negative, with a plus sign (+) at the positive end of the arrow. This symbolism is used when we wish to emphasize that the polar bond has an electrical dipole. The magnitude of the dipole is expressed by the dipole moment μ (μ = e × d where e is the charge and d is the charge separation). 

The dipole moment of a bond is called a bond moment, and the dipole moment of a molecule is the vector sum of the bond moments involved.