Geoscience Reference
In-Depth Information
natural isotopes of naturally occurring elements are stable, i.e. the number of their protons
and neutrons remains unchanged, simply because most radioactive isotopes have vanished
over the course of geological time. They are therefore not a danger to people.
1.1 The periodic table
The atomic number of an element is equal to the number of its protons. We have seen before
that the atom's mass number is equal to the number of particles making up its nucleus. The
Avogadro number
is the number of atoms contained in 12 g of the carbon-12 isotope.
The atomic mass of an isotope is the weight of a number
N
of atoms of that isotope.
Dimitri Mendeleev's great discovery in 1871 was to demonstrate the periodic character of
the properties of elements when ordered by ascending atomic number (
Fig. 1.2
). Melting
point, energy of formation, atomic radius, and first ionization energy all vary periodically as
we work through Mendeleev's table. The geochemical properties of elements are reflected
by their position in this table. The alkali metals (Li, Na, K, Rb, Cs), alkaline-earth metals
(Be, Mg, Ca, Sr, Ba), titanium group elements (Ti, Zr, Hf), but also the halogens (F, Cl,
Br, I), inert gases (He, Ne, Ar, Kr, Xe), rare-earths (lanthanides), or actinides (uranium
family) all form groups sharing similar chemical properties; these properties are indeed
sometimes so similar that it was long a challenge to isolate chemically pure forms of
some elements such as hafnium (Hf), which was only separated from zirconium (Zr) and
identified in 1922.
N
I
II
III
IV
V
VI
VII
VIII
1
2
H
He
3
4
5
6
7
8
9
10
Li
Be
B
C
N
O
F
Ne
11
12
13
14
15
16
17
18
Na
Mg
Al
Si
P
S
l
r
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
K
a c i
V
r
Mn
Fe
Co
Ni
Cu
Zn
Ga
Ge
As
Se
Br
Kr
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
Rb
Sr
Y
Zr
Nb
Mo
(Tc)
Ru
Rh
Pd
Ag
Cd
In
Sn
Sb
Te
I
Xe
55
56
57
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
La*
Cs
Ba
Hf
Ta
W
Re
Os
Ir
Pt
Au
Hg
Tl
Pb
Bi
Po
At
Rn
87
88
89
Ac**
Fr
Ra
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
*Lanthanides
La
Ce
Pr
Nd
(Pm) Sm
Eu
Gd
Tb
Dy
Ho
Er
Tm
Yb
Lu
89
90
91
92
93
94
**Actinides
Ac
Th
Pa
U
Np
Pu
Os
Siderophile
Cu
Chalcophile
Rb
Lithophile
N
Atmophile
Figure 1.2
Mendeleev's periodic table of the elements and their geochemical classification after
Goldschmidt. The elements in parentheses do not occur naturally on Earth. The atomic number
of each element is given. Roman numerals over columns indicate groups.
