WHAT WE CAN LEARN FROM THE PERIODIC TABLE - Chemical Fundamentals of Geology and Environmental Geoscience - page 103

Electronegativity

Sc to Zn (zinc) comprise the First Transition Series . They
include many industrially important metals (Figure 9.7).
The next six elements, from gallium (Ga) to the noble
gas krypton (Kr), correspond to the filling of the 4p
orbitals, the next subshell in order of energy (Figure 5.7).
Subsequent periods follow the same pattern, except
for the added complication of the availability of 4f
and 5f orbitals in very heavy atoms. The entry of elec-
trons into 4f orbitals generates a series of geochem-
ically important trace elements called the lanthanides
( cerium, Ce, to lutetium, Lu), although in geological
literature they are more commonly known - together
with the preceding element lanthanum, La - as the
rare earth elements ( REEs ; see Chapter 9).
In geological terms, the most important heavy
elements of all are thorium (Th) and uranium (U).
Owing to their radioactivity, they make a major contri-
bution to heat generation in the Earth, and are impor-
tant in geochronology. They belong to a similar series
of elements arising from the filling of 5f orbitals, known
as the actinides ( Chapter 9). None have stable nuclei,
only Th and U having sufficiently long half-lives to
occur in nature today (Chapter 10).

Ionization energy provides a useful indication of the
periodicity of element behaviour, but its chemical
applications are limited to elements that lose electrons
to form cations. Electronegativity is a more versatile
concept, summarizing the chemistry of all kinds of
elements and the bonds they form. Electronegativity is
a number that indicates the capacity of an atom in a
molecule or crystal to attract extra electrons . In the alkali
metals, this capacity is hardly developed at all; elements
that tend to give electrons away rather than attracting
additional ones are called electropositive elements. They
appear on the left-hand side of the Periodic Table.
Their electronegativity values are low, beginning at 0.8
(for the alkali metals K, Rb and Cs).
The most electronegative elements are those having
nearly complete valence shells, on the right-hand side of
the Periodic Table (Figure 6.3). The nuclear charge effect
draws the valence orbitals closer to the nucleus, offering
an incoming electron a state of low energy, compared
for example with the valence shell of an alkali metal
where the attraction of the nucleus is only weakly felt.

Figure 6.3 Variation of electronegativity (height of each block) across the Periodic Table (stable elements only). Groups and
periods are shown in Roman and Arabic numerals respectively. Data from Henderson and Henderson (2009) and other
sources. The gap between molybdenum (Mo) and ruthenium (Ru) belongs to the radioactive element technetium
(Tc - see Exercise 6.4).