SOME GEOLOGICALLY IMPORTANT ELEMENTS - Chemical Fundamentals of Geology and Environmental Geoscience - page 171

derivatives in which every hydrogen has been replaced
by chlorine or fluorine atoms. Whereas elemental
chlorine is highly reactive in the atmosphere and is
therefore rapidly washed out by rain, volatile CFCs
may persist for up to a hundred years (Table 9.7.1).
They are very powerful 'greenhouse gases' (Box 9.7),
but their persistence once posed a more immediate
threat through their capacity to deplete stratospheric
ozone: their atmospheric residence time is long enough
to allow them to penetrate the stratosphere, where
they undergo photochemical decomposition and
release Cl radicals that destroy ozone. Following
implementation of the Montreal Protocol in 1989, how-
ever, CFCs have been replaced for most industrial uses
by less persistent hydrochlorofluorocarbons (HCFCs);
these pose less threat to stratospheric ozone, although
their global warming potential is only slightly less than
CFCs (Table 9.7.1).
The rarer halogens, bromine (Br, a liquid at room
temperature) and iodine (I, a solid), react in a similar
way to chlorine.

Helium consists of two stable isotopes, 3 He and 4 He.
In natural helium escaping through the Earth's surface
(recovered from oilfield brines or hot springs, for
example), 3 He is a relic of the He originally incorpor-
ated during the accretion of the Earth (Chapter 10),
whereas 4 He - about a million times more abundant -
is almost entirely the accumulated product of the
radioactive decay of thorium (Th) and uranium (U)
throughout the Earth's history.
The geochemist's principal interest in Ar is for radio-
metric dating, using the K-Ar method (Chapter 10) or
the more recently developed 39 Ar- 40 Ar method
(Box 10.2). As to environmental impact, the radioactive
inert gas radon (Rn) poses significant concern (Box 9.9).

Transition metals

Most metals that we use in the home, in the office and
in industry are transition metals , making up the d-block
of the Periodic Table. The essential feature of a trans-
ition metal is the presence in the atom or ion of a part-
ially filled d subshell (Chapter 6). The d-block can be
divided into first, second and third transition series
(Figure 9.7), according to whether the 3d, 4d or 5d sub-
shell is the partially filled subshell. The elements of
Group IIb (zinc, cadmium and mercury), as they have
full d subshells, are not formally classified as transition
metals.
The transition metals share a number of important
chemical characteristics.

Noble gases

The elements helium
(He), neon (Ne), argon
(Ar), krypton (Kr), xenon
(Xe) and radon (Rn - see
Box 9.9), found in col-
umn 18 at the right-hand
side of the Periodic Table,
have completely filled
valence shells. This elec-
tronic structure is so stable that these elements display
negligible chemical reactivity, and exist (except at
extremely low temperatures, Box 7.7) as monatomic
gases. They are known as the noble , inert or rare gases .
Although helium is the second most abundant
element in the universe after hydrogen (Figure 11.2), it
comprises only 0.00052% of the Earth's atmosphere.
Unlike other gaseous elements like N 2 , helium has not
accumulated in the atmosphere because its relative
atomic mass is too low for it to be retained by the
Earth's gravitational field: the escape velocity of
helium atoms (and H 2 molecules) is well below the
actual average thermal velocity of such atoms at
normal temperatures.

He

B C N O F

Ne

Al Si P S Cl

Ar

Ga Ge As Se Br Kr

ln Sn Sb Te l Xe

(a) Most transition metals and their alloys are tough,
chemically stable metals that have innumerable
industrial and domestic uses (Figure 9.7).
(b) Most can utilize more than one oxidation state
(valency) in geological environments. The most
familiar example is iron (Box 4.7). Figure 9.8 shows
the range of oxidation states among the first
transition series. In the elements up to manganese,
all d electrons are able to participate in bonding,
and high oxidation states can therefore be attained.
In elements like iron, cobalt (Co) and nickel (Ni),
the d subshell behaves more like the electron core
(Chapter 5): only the 4s electrons and perhaps
one of the 3d electrons have energies high enough
to be used in bonding, and only low oxidation
states occur.

Tl Pb Bi Po At Rn