ion polarization

Picture a negative ion as a soft, squishy balloon of electrons around its nucleus. Now bring a small, strongly positive ion close. The positive charge tugs on the anion's electron cloud, pulling it out of shape — distorting it toward the cation. This distortion is ion polarization, and the more the cloud is pulled into the space between the two ions, the more the bond starts to look like shared electrons, that is, covalent.

Two properties govern this. Polarising power is the ability of a cation to distort a neighbour's electron cloud; it is large for cations that are small and highly charged (high charge density), and for cations without a noble-gas core (such as d10 ions like Zn2+, Cd2+, whose d electrons shield the nucleus poorly). Polarisability is how easily an anion's cloud is distorted; it is large for anions that are big and highly charged, because their outer electrons are far from the nucleus and loosely held. Strong polarising cation plus easily polarisable anion equals significant covalent character laid over an ionic skeleton.

Ion polarization is the mechanism that sits behind Fajans' rules, and it explains many real inorganic facts: the deepening colours of silver halides AgCl to AgI (greater polarisation shifts light absorption), the low solubility and partly covalent bonding of heavy-metal sulfides, and why polarising cations destabilise large anions like carbonate, lowering the decomposition temperatures of the heavier group-2 carbonates. It is a single, physically clear idea — distort the electron cloud — that bridges the ionic and covalent extremes.