Michael S. answered • 12d
B.S. in Chemistry, Indiana University; Organic Chem Teaching Intern
Cesium is more polarizable.
Polarizability measures how easily an external electric field distorts an atom's electron cloud. So the question is really: whose outermost electron is held most loosely?
The reason
Both are group 1 metals with a single valence electron, but they sit in different periods:
K is [Ar] 4s1
Cs is [Xe] 6s1
Cesium's valence electron occupies a 6s orbital, two shells further out. Two things follow, and they push the same way:
Distance. The 6s electron sits much farther from the nucleus, and the electrostatic pull falls off with distance. Atomic radius is about 227 pm for K and about 265 pm for Cs.
Shielding. Cesium has 54 core electrons between its valence electron and the nucleus, against 18 for potassium. Those cores screen the nuclear charge so effectively that the effective nuclear charge felt by the valence electron is roughly the same in both - but it is acting over a much greater distance in cesium.
A loosely held, far-out electron cloud is a soft, squishy one, and a soft electron cloud distorts easily. That is polarizability.
The numbers, if you want them
Atomic polarizability volume is roughly 43 cubic angstroms for potassium and roughly 59 for cesium - about 40 percent larger. Cesium in fact has the highest atomic polarizability of any element in the periodic table, which is the same reason it has the lowest ionization energy of any stable element.
The general rule to carry away
Polarizability increases going DOWN a group and DECREASES going left to right across a period. Down a group the valence shell gets bigger and looser; across a period the nuclear charge rises while electrons enter the same shell, pulling the cloud in tight. Big and diffuse means polarizable; small and compact means not.
The same reasoning applies to the ions: Cs+ is more polarizable than K+, and among anions I- is far more polarizable than F-.
Why anyone cares
Polarizability is what makes London dispersion forces strong, so it drives boiling points across a series of similar substances - it is why I2 is a solid and F2 a gas at room temperature.
It also governs covalent character in ionic compounds. Fajans' rules say that a small, highly charged cation paired with a large, polarizable anion pulls that anion's cloud out of shape and gives the bond real covalent character. That is why AgI behaves far less like a simple ionic salt than AgF does, and it is usually the reason a course introduces polarizability in the first place.
