Page 162 - Academic Press Encyclopedia of Physical Science and Technology 3rd InOrganic Chemistry

P1: GPA Final Pages
Encyclopedia of Physical Science and Technology EN007D-343 July 10, 2001 20:13

820 Inorganic Exotic Molecules

to lions to lakes that surround us do not collapse. Atoms and astro-physicists who study these and other exotic
have size. Molecules have size and shape. Despite this dif- (both low- and high-temperature) phenomena involving
ference in heft, locality, and localizability of protons and helium.
electrons, their charge ratio is seemingly precisely 1: −1.
This allows for discrete elements: there is no element that
interpolates oxygen and sulfur to allow us to understand D. Lithium
better the plethora of species containing these two ele- We close our discussion of atoms with lithium. Its three
ments in the same group in the periodic table. It allows for electrons do not result in even greater inertness. It does
neutral atoms and molecules as well as those with discrete, not have a 1s electron configuration, but rather 1s 2s ,
2
3
1
integer charges. There is no element that interpolates car- and so more resembles atomic hydrogen than it does he-
bon and nitrogen to allow us to understand species with lium because of the half-filled, singly occupied valence
these elements. level s orbital. We can thank, or alternatively blame,
All neutral atoms and molecules have singly positively the Pauli exclusion principle for this, and will consider
charged counterparts from the loss of an electron, some this principle, like the conservation of charge (and the
(but, most assuredly, not all) form negatively charged conservation of mass and of atom types in a chemical
counterparts by the gain of an electron, and some form reaction), absolutely inviolate. It is easy to forget how
multiply positive or negative counterparts. Atomic multi- strange all of these rules are, even if, by always assuming
ple positive ions, in isolation, are “stuck” that way. Atomic their validity, we must turn to more complicated species
multiple negative ions, in isolation, are always unstable to be labeled exotic in the discussion that follows. For
relative to loss of an electron. In isolation, multiply pos- example, we will see that lithium and hydrogen chem-
itive and multiply negative ions very often are unstable istry are very distinct, perhaps starting with the sim-
with regard to bond cleavage to minimize charge repul- ple observation that elemental lithium is normally found
sion. This instability is not what we may recall from our as a solid metal and elemental hydrogen is a diatomic
“chemical childhood.” Then again, the study of atomic gas.
and molecular matter in isolation is very often a study
of “exotica,” however common the species when in solu-
tion or as a pure liquid or solid. We will return to these II. HYDROGEN AND THE ALKALI METALS
ions later and simply ask now why any neutral atom or
molecule that definitionally has an equal, and therefore
A. Univalent Cations, Acids, and Bases
balanced, number of protons and electrons would “want”
to add another electron. Suffice it to say, we may view all The normal oxidation state for hydrogen and the alkali
chemical matter as exotic. We will not, clearly we cannot, metals is +1, and indeed their unipositive cations have
discuss the almost 20 million known chemical species, or long been known to the chemical community. It is not
even the sizable minority that are called inorganic or even by accident that these elements are called group 1 or
organometallic. group 1A. The alkali metal cations abound in solid (and
molten) salts and in solution. By contrast, H exists only
+
as complexes, where its bonding with the molecules of the
C. Helium
condensed phase (i.e., solid, liquid, solution) media is so
Consider helium. Atomic helium with its most stable nu- strong that one should not refer to this monatomic cation
clear and electronic arrangement of two protons, neutrons in beginning texts, but instead to quite common ions such
+
and electrons, offers few surprises, not even its phenom- as [H 3 O] and much more exotic ones such as [H 2 F] .
+
2
enal inertness because of the 1s electron configuration. (The former polyatomic ion is also a relatively common
Despite the exotica in Section VIII.B, helium normally ion in salts, although these salts are also, and generally
occurs simply as the atom, as He, and not as the dimer, better, describable as hydrates of strong acids.) That these
trimer, or any oligomer. Likewise, save in stellar interiors, two ions are isoelectronic and isostructural with NH 3 and
+
helium fails to trimerize to form carbon, or four higher H 2 O, respectively, does not make [H 2 F] less exotic—
oligomers that may be recognized as oxygen, neon, mag- perhaps this is because we usually think of HF as an acid
nesium, silicon, or sulfur: these processes are, in fact, and not as a base. In nonaqueous media, HF is both. In
highly exothermic This is perhaps no surprise—there is gaseous media, HF is both an acid and base also, but we
a high electrostatic (Coulomb, charge repulsion) barrier know of no neutral hydrogen-containing species that is
to these reactions—but had this not been so, chemistry not an acid, and no neutral species at all that is not a base:
as we know it would not exist. Certainly, we chemists we recognize H and [H 3 ] as the conjugate base and
−
+
would not exist nor would the condensed-matter, nuclear, conjugate acid of the well known H 2 .