An Introduction to Environmental Chemistry

is required to remove these electrons due to the increasing nuclear attraction.

This means that aluminium will form trivalent cations whereas silicon typically

will not: instead it shares its electrons in covalent bonds (Section 2.3), except in

one special case (Section 2.3.2). At the top of each energy ramp are the elements

He, Ne and Ar that cling tenaciously to all of their electrons. These elements

have no valence electrons and therefore no significant chemical reactivity. These

chemically inert elements are often called the inert or noble gases and form

column O on the far right of the Periodic Table (Fig. 2.2).

Although the periodic pattern becomes more complicated above Zvalues of

20, the overall ordering persists. Complications arise in the so-called transition

elements that occupy a position between columns II and III of the Periodic Table

(Fig. 2.2). These elements have between one and three valence electrons. Impor-

tantly, however, the electrons in the orbital below the valence electrons have

almost the same energy as the valence electrons themselves. In some compounds,

Environmental Chemist’s Toolbox 17

1 2 3 4 5 6 7

Period

Alkali metals
Alkali earth metals Lanthanides*

Actinides

Short

periods

Long

periods

Inert (noble) gases

Halogens

Transition elements

Metals

Semi-metals

Non-metals

H

1

Li

3

Be

4

Na

11

Mg

12

K

19

Ca

20

Rb

37

Sr

38

Cs

55

Ba

56

Fr

87

Ra

88

Ac

89

Sc

21

Y

39

La

57

Ti

22

Zr

40

Hf

72

V

23

Nb

41

Ta

73

Cr

24

Mo

42

W

74

Mn

25

Tc

43

Re

75

Fe

26

Ru

44

Os

76

Co

27

Rh

45

Ir

77

Ni

28

Pd

46

Pt

78

Cu

29

Ag

47

Au

79

Zn

30

Cd

48

Hg

80

Ga

31

In

49

Ti

81

Al

13

Ge

32

Sn

50

Pb

82

Sb

51

Bi

83

Po

84

At

85

Rn

86

Te

52

B

5

C

6

N

7

O

8

Si

14

P

15

S

16

As

33

Se

34

I

53

Xe

54

Br

35

Kr

36

Cl

17

Ar

18

F

9

Ne

10

He

2

Ce

58

Th

90

Pa

91

Pr

59

Nd

60

U

92

Pm

61

Sm

62

Eu

63

Gd

64

Tb

65

Dy

66

Ho

67

Er

68

Tm

69

Yb

70

Lu

71

Super-heavy elements – not naturally occurring

Ia IIa IIIa IVa Va VIa VIIa VIII OVIIbVIbVbIVbIIIbIIbIb

a subgroups b subgroups

Fig. 2.2Periodic Table of the elements and their Znumbers. Note that the periodic pattern is complicated by

the transition metals between columns II and III. *La and the lanthanides are known as the rare earth elements

(REE). The table has been constructed using conventional terminology and further details can be found in

basic chemistry textbooks. Gill (1996) gives an accessible summary with a strong applied earth science stance.

Elements in bold are those most abundant in environmental materials (see Fig. 2.3). After Gill (1996), with

kind permission of Kluwer Academic Publishers.