Polar Character of Covalent Bonds and Fajans' Rules

No real bond between two different atoms is ever purely covalent (perfectly equal sharing) or purely ionic (complete electron transfer) — both are idealised limits, and most actual bonds lie somewhere on a continuum between them.

Polar covalent bonds. When two covalently bonded atoms differ in electronegativity — their intrinsic tendency to attract the shared electron pair — the bonding pair is pulled closer to the more electronegative atom. That atom develops a partial negative charge ($\delta^-$) and the less electronegative atom a partial positive charge ($\delta^+$), creating a bond dipole. Hydrogen chloride, $\text{H}-\text{Cl}$, is a standard example: chlorine is considerably more electronegative than hydrogen, so the bonding pair sits closer to chlorine, giving $\overset{\delta^+}{\text{H}}-\overset{\delta^-}{\text{Cl}}$. The larger the electronegativity difference between the two atoms, the more polar the bond, and in the extreme limit of a very large electronegativity difference, the bond becomes essentially ionic rather than merely polar covalent.

Fajans' rules — covalent character of ionic bonds. The reverse effect operates on bonds that are nominally ionic: the cation's positive charge can attract and distort (polarise) the electron cloud of the neighbouring anion, pulling electron density back toward itself and giving the "ionic" bond some genuine covalent (shared-electron) character. Kazimierz Fajans formulated a set of rules predicting when this polarisation, and hence covalent character, will be most pronounced:

• Small cation, large anion. A small, compact cation concentrates its positive charge over a small volume, producing an intense electric field capable of strongly polarising a neighbouring ion; a large anion has its outer electrons held more loosely and further from its own nucleus, making that electron cloud easier to distort.
• High charge on either ion. A more highly charged cation polarises more strongly, and a more highly charged anion is itself more easily polarised.
• Cations with a pseudo-noble-gas (18-electron) configuration, such as $\text{Zn}^{2+}$, polarise anions more strongly than cations of similar size and charge with a simple noble-gas (8-electron) configuration. …