Polarizing Power of Common Cations, in (Charge)/(Ionic Radius in Å)2
Li+
1.7
Na+
1.0
K+
0.5
Rb+
0.4
Cs+
0.4
Be2+
10
Mg2+
3.9
Ca2+
2.0
Sr2+
1.5
Ba2+
1.1
Sc3+
5.4
Ti3+
6.7
V2+
4.9
Cr2+ 3.8
Cr3+ 7.9
Mn2+ Fe2+ 3.3
4.5
Fe3+ 9.9
Co2+ 3.6
Co3+ 10
Interpretation
Spectator ion. Negligible Lewis Acidity to water.
Very weak aqueous interactions.
2.1-5.0
Strongly polarizing. Notable increase in acidity of
aqueous solutions. Significant covalent bonding
Very polarizing and covalent.
Describing these as ions is problematic.
Oh, give it up. That’s not an ion.
> 10
Cu+ 1.7
Cu2+ 3.8
Ag+ 0.8
Au+ 0.5
Au3+ 4.1
Polarizing Power
0.0-1.0
1.1-2.0
5.1-10
Ni2+
4.2
Zn2+
3.7
Cd2+
2.2
Hg+ 0.7
Hg2+ 1.9
Al3+
10
Ga3+
7.8
In3+
4.7
Tl+
0.4
Sn2+ 1.4
Sn4+ 8.4
Pb2+ 1.4
Pb4+ 6.7
Solubility
Generally very soluble with all anions*
Generally soluble, solubility limited with small -1
ions.† Insoluble with -2 and -3 ions.
Somewhat soluble with -1 anions, insoluble with small
-1 anions (OH-, F-) and -2 and -3 anions.
Somewhat soluble (if at all) only with large -1 anions.
Often unstable in solution.
Generally insoluble unless a molecular substance.
*Note that the “soft” purple and blue ions to the right of the periodic table are almost always insoluble in their interactions with soft
anions (S2-, Cl-, Br-, I-, SO42-, to a lesser extent CO32-) because they form good soft-soft covalent interactions and are poorly solvated
by water. They therefore act as exceptions to the general rule “If one or both ions are + or -1, it’s soluble, otherwise it’s not”.
†
Small ions: Because of their small size, these act as if they were one charge higher: OH-, F- (both act like -2), O2- (acts like -3)
The cutoff for this chart was a ratio > 10, which is why Si4+, B3+, etc. are not included. These species bind very covalently.