WHAT WE CAN LEARN FROM THE PERIODIC TABLE - Chemical Fundamentals of Geology and Environmental Geoscience - page 104

Electronegative atoms thus have the power to attract
and retain additional electrons, in spite of the net nega-
tive charge that the atom thereby acquires (making it an
anion ). The explanation for this phenomenon is partly
wave-mechanical, to do with the special stability of the
noble-gas configuration that such an atom can achieve
with the aid of borrowed electrons. The most electron-
egative element is fluorine (F, electronegativity 4.0).
Figure 6.3 shows that electronegativity varies in a
fairly regular manner in the Periodic Table. It increases
strongly from left to right, and more gently from bottom
to top (although the latter trend is reversed in the central
area). As a rule, metals have electronegativities less than
2.0, whereas non-metals have values greater than 2.5.

Many of the elements in the central parts of the
Periodic Table behave in a more complicated manner.
In Chapter 4 we discussed a few elements that can
adopt one of several oxidation states depending on the
oxidizing or reducing properties of the environment.
Each of these oxidation states represents a separate
valency of the element concerned. Multiple valency
is most characteristic of the transition elements
(Figure 9.7). The best-known example is iron, which in
addition to the metallic state (valency 0) can exist in the
geological world in ferrous (divalent) or ferric (triva-
lent) compounds. It often happens - as in the case of
iron - that none of the oxidation states corresponds to
the valency suggested by the element's position in the
Periodic Table. Iron occurs in Group VIII (Box 6.2,
Figure 6.3), but exhibits valencies of 2 and 3. It is clear
that such elements do not utilize all of the electrons that
nominally belong to the valence shell in forming bonds,
for reasons that will be examined in Chapter 9. The
same is true of many heavier elements in the p-block
(Box 6.2.). Tin, for instance, occurs in the geological

Valency

The number of bonds that an atom can form as part of
a compound is expressed by the valency of the element.
In chemical reactions, elements adjust their electron
populations to achieve a noble-gas configuration.
Because the single valence electron of alkali metals
like Na (1s 2 2s 2 2 6 3s 1 ) allows them to form only one
bond (Chapter 7), they are said to be monovalent
(valency = 1). Magnesium (1s 2 2s 2 2 6 3s 2 ) has a valency of
two (it is divalent ), because two of the electrons in the
neutral Mg atom are in the valence shell, and can be
used to establish bonds. For strongly electropositive
elements like these (Figure 6.3), valency is equal to the
number of electrons in the valence shell, and can there-
fore be determined from the column in the Periodic
Table in which the element occurs. The valencies of the
elements B and Al ( trivalent ), C and Si (quadrivalent)
and P (pentavalent) also conform to their position in
the Periodic Table.
The electronegative elements like oxygen and chlor-
ine require a complementary definition of valency.
These are elements with nearly complete valence
shells. Because they can achieve a noble-gas electron
structure by accepting extra electrons from other atoms
(or sharing them), it is the number of vacancies in the
valence shell that determines the number of bonds that
can be formed. Oxygen, with six valence electrons and
two vacancies, can establish two bonds with other
atoms and is divalent. Chlorine (seven valence elec-
trons, one vacancy) needs only one electron to com-
plete the valence shell and is therefore monovalent.

Box 6.2 Sub-groups and blocks

the condensed form of the periodic table for the first 20
elements (Figure 6.2) is easy to divide into eight col-
umns, but the introduction of the transition series - an
additional ten columns - makes necessary a revision of
these column headings. In order that elements like B, C
and al retain group numbers reflecting their valency,
groups are divided into 'a' and 'b' subgroups as shown
in Figure 6.3 and on the inside rear cover, leaving three
columns in the middle of the transition series (headed by
Fe, Co and Ni) which are lumped together as Group VIII.
Formal chemical similarities exist between correspond-
ing a and b subgroups (e.g. valency, as in subgroups IVa
and IVb), but they are generally outweighed by the differ-
ences in electronegativity (Figure 6.3).
It is useful to divide the periodic table into blocks
reflecting the kind of subshell (s, p, d or f) that is cur-
rently incomplete or just filled. the transition series
comprises the d-block and the lanthanides and acti-
nides the f-block. It is important to recognize the direct
relationship of these blocks, and of the structure of the
periodic table as a whole, to the energy-level diagram
shown in Figure 5.7.