Linus Pauling: Many properties of substances can be discussed in terms of the sizes of the ions. One example is the formula of hydrates.
Sodium chloride doesn’t form hydrates ordinarily, out of aqueous solutions, salt crystallizes as anhydrous NaCl. The attraction
of the sodium ion and chloride ion for water molecules is not very great. But the ions with larger electric charge, the bivalent
ions, beryllium, magnesium, calcium, usually crystallize out of aqueous, their salts crystallize with water of crystallization,
and the water of crystallization is, to some extent, predictable. For example, beryllium is a small ion, magnesium is a larger
ion.
The size of magnesium relative to the size of a water molecule is about the same as sodium relative to chlorine. We would
expect then that a magnesium ion would coordinate six water molecules, about itself at the corners of an octahedron and, in
fact, magnesium chloride crystallizes with formula MgCl2 6H2O. Many of the salts of magnesium appear as crystallized from aqueous solution with six molecules of water which, without
doubt, in fact, we know from x-ray diffraction experiment, are arranged octahedrally about the magnesium ion.
The beryllium ion is smaller, so small that it can fit inside a tetrahedron of four water molecules arranged in this way,
and beryllium sulfate crystallizes as BeSO4 4H2O, the four water molecules being arranged in this tetrahedral manner around the beryllium ion.
In addition to the metals that are close to the noble gases in the Periodic Table - lithium and beryllium close to helium,
sodium and magnesium close to neon, and so on - and that easily lose one or two or three electrons to become positive ions,
there are also a number of other metals, those in the transition groups that easily lose electrons to become cations. For
example, chromium, manganese, iron, cobalt, nickel, copper, zinc appear in most of their compounds as cations with charge
plus two or plus three.
Zinc, in the period 2b of the Periodic Table, zinc forms the ion Zn++, Zn double plus. With atomic number thirty, it loses two electrons easily to form the zinc ion which has twenty-eight electrons.
Now, eighteen of these twenty-eight electrons are in the shells up to argon, completed argon structure, ten more occupy, ten
more are present in the krypton shell. They are just enough to occupy the five orbitals of the 3d sub shell.
Copper, when it loses one electron, to form the cuprous ion, has also achieved to be the structure in which there are ten
electrons in the five 3d orbitals. But of course, this state, this state with valency one, charge plus one, is not the most
stable one for copper. Most copper salts such as copper sulfate, blue vitriol, CuSo4 5H2O, most copper salts involve copper that has lost two electrons and there is no simple explanation of the tendency of copper
to lost two electrons.
The most that we can say for these metals in the transition series is that they lose one electron easily. The second electron
is pulled off with greater difficulty because one is removing an electron from an ion that already has a positive charge that
is pulling the electron back and sometimes it is possible also to remove a third electron. These elements in the transition
periods usually turn up with ions that have charge plus two or plus three. And, although it is possible to get some sort of
understanding of why one, understanding of why one charge plus two or plus three is the more stable for nickel, say, and another,
the more stable for cobalt. This is not a simple branch of chemical theory.
