Mendeleev, polarizability, Fajan's rules

Dmitri Mendeleev devised the periodic table in 1869.

Dmitri Mendeleev (1834 - 1907)

The polarizability of an atom is the ability of its electron cloud to be distorted by an electric field, such as by neighboring ions. It usually denoted as α. Bonds which have a heavily polarized electron cloud between them are considered covalent.

As a chemist you may come across two ions and be expected to have some intuition about whether their bonding would be ionic or covalent, factors which affect this are summarized in Fajan's rules:

1. Small, highly charged cations have polarizing ability.
2. Large, highly charged anions are easily polarized.
3. Cations that do not have a noble-gas electron configuration are easily polarized.

One analogy for the first rule is trying to suck up water from a lake with a vacuum cleaner. You'd create more of a distortion by using a narrow tip to focus on a small area.

For the second rule we should remember that large anions usually have high principle quantum numbers. The gap between energy levels decreases as we increase n:

The textbook writes that polarizability is high when separation between frontier orbitals is low. I suppose electrons have an easier time moving through multiple small energy gaps then one large gap. Intuition for this may be found in the equations for quantum tunneling, where the chance of an electron passing through a barrier falls very rapidly as the length of the barrier is increased.

For the third rule, it makes sense that electrons in a more stable configuration are harder to drag away.