Geology Reference
In-Depth Information
bonding and - with the exception of Be (Box 9.3) - read-
ily form the doubly charged M 2+ cation.
Boron (B) and aluminium (Al), to the right of Be and
Mg in Figure 6.1b, each have three valence electrons to
utilize in their chemical reactions; and carbon (C) and
silicon (Si) each have four. The increasing nuclear
attraction makes these electrons progressively harder
to remove, however, and the tendency among these
elements to form cations is significant only for Al; the
chemistry of B, C and Si is dominated by bonds in
which electrons are shared (Chapter 7).
The periodic pattern becomes more complicated at Z
values above 20 (inset in Figure  6.1b), owing to the
presence of electrons in d-orbitals. The overall periodic-
ity nevertheless persists, with the minimum ionization
energies belonging to the alkali metals rubidium (Rb)
and caesium (Cs), and the maximum values coinciding
with the noble gases krypton (Kr) and xenon (Xe).
highlights their common chemical properties. This is
shown for the first three periods (including hydrogen
and helium) in Figure 6.2. Notice that each of the groups
of similar elements that emerged from Figure 6.1b now
forms its own column. The first contains the alkali
metals Li, Na and K, to which by tradition we add the
element hydrogen, because it resembles the alkali met-
als in having just one electron in the valence shell. In
column 2 we find the alkaline earth metals, Be, Mg and
Ca; in column 3, B and Al, and so on.
Numbering the columns in Figure  6.2 from left to
right divides the elements into eight groups, identified
by Roman numerals, according to the number of elec-
trons in the valence shell. Numbering the periods indi-
cates the n -value (the principal quantum number) of
the current valence shell: thus for Al in period 3 the
valence electrons are in orbitals that have n = 3 (the M
shell). The noble gases at the end of each period mark
the closure (filling up) of the p-orbitals of that shell.
It is therefore logical to place helium at the head of
this eighth column rather than column 2, as its two
electrons close the K (1 s) shell.
This is the rationale of the Periodic Table , which is
shown in its complete form on the inside cover. Notice
that it has been split between Groups II and III in order
to accommodate elements like scandium (Sc), titanium
(Ti) and iron (Fe). This is where electrons begin to occupy
the lowest-energy d-orbitals (3d). The ten elements from
The Periodic Table
Having arranged the elements in order of atomic num-
ber, they can be divided into several periods , each one
beginning with an alkali metal - except the first, which
begins with hydrogen - and concluding with a noble gas.
Writing successive periods underneath each other on
a page gives a coherent layout of the elements that
Columns (groups)
I
II
III
IV
V I
VII
0
Atomic number
1 H
2 He
1
Element
symbol
3 Li
4 Be
5 B
6 C
7 N
8 O
9 F
10 Ne
2
11 Na
12 Mg
13 Al
14 Si
15 P
16 S
17 Cl
18 Ar
3
19 K
20 Ca
4
Figure 6.2 A condensed Periodic Table for the first 20 elements. Chemical symbolism is discussed in Box 6.1.
 
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