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
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
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.