The Definition of Heavy Metals in Plant Science 221 Metals

Metals are often characterised and distinguished from nonmetals by their physical properties - the ability to conduct heat, and an electrical resistance that is directly proportional to temperature, malleability, ductility and even lustre (Housecroft and Sharpe 2008; Müller 2007). These properties, especially that of a temperature-dependent conductivity, at least allow us to define what a metal is in contrast to

Friedrich-Schiller University of Jena, Institute for General Botany and Plant Physiology, Dornburgerstr 159, 07743, Jena, Germany e-mail: [email protected]e

I. Sherameti and A. Varma (eds.), Soil Heavy Metals, Soil Biology, Vol 19, DOI 10.1007/978-3-642-02436-8_2, © Springer-Verlag Berlin Heidelberg 2010

nonmetals and metalloids. However as mentioned above, all of these physical properties are lost after the metal has been chemically transformed into a chemical compound that can be taken up by plants (Shaw et al. 2004). It is well known that the properties of chemical elements can be determined from their positions in the periodic table of the elements (Fig. 2.1). In general, the chemical elements become more metallic as we move towards the lower left corner of the table and nonmetal-lic towards the upper right corner. In other words, metallic character decreases from left to right and from the bottom to the top of the table. Metalloids (elements with properties intermediate between metals and nonmetals) occur close to the diagonal border between metals and nonmetals in the table. A metal can be categorised according to the last electronic subshell in its atom. There are s-elements, which can be subdivided into alkaline elements (first main group) and alkaline earth elements (second main group). All s-elements are metals except for H (the first element in the first main group). The first element in the second main group, Be, is also somewhat special (its oxides are amphoteric), but it is still considered to be a metal. Among the other groups of the periodic table, d-group elements (transition elements) are all metals. Many of them form compounds with different valence states, which is an important factor in their toxicity. Some of the oxides of transition elements have slightly amphoteric properties, but they are still all considered to be metals. Then there are the f-group elements, also known as the rare earth elements, which are subdivided into the lanthanide series (including La) and the actinide series (including Ac). All of these rare earth elements are also metals and so are sometimes called rare earth metals. The next group, the p-group, occurs towards the right hand side of the periodic table and thus represents a mixed group of

Alkali elements la

Alkali elements la

1

Alkali earth elements

2

lla

llla

lVa

Va

Vla

Vlla

He

3

4

5

6

7

8

9

10

Li

Be

Lead

B

C

N

O

F

Ne

11

12

Transition elements

group

13

14

15

16

17

18

Na

Mg

lllb

IVb

Vb

Vlb

Vllb

Vlllb

lb

llb

Al

Si

P

S

Cl

Ar

19

20

21

22

23

24

25

26

27

28

29

30

31

32

33

34

35

36

K

Ca

Sc

Ti

Y

Cr

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

ln

Sn

Sb

Te

l

Xe

55

56

57

72

73

74

75

76

77

78

79

80

81

82

83

84

85

86

Cs

Ba

La

Hf

Ta

W

Re

Os

Ir

Pt

Au

Hg

T1

Pb

Bi

Po

At

Rn

87

88

89

104

105

106

107

108

109

110

111

112

113

114

115

116

Fr

Ra

Ac

Rf

Db

Sg

Bh

Hs

Mt

Ds

Rg

Lanthanide

57

58

59

60

61

62

63

64

65

66

67

68

69

70

71

La

Ce

Pr

Nd

Pm

Sm

Eu

Gd

Tb

Dy

Ho

Er

Tm

Yb

Lu

Actinide

89

90

91

92

93

94

95

96

97

98

99

100

101

102

103

Ac

Th

Pa

U

Np

Pu

Am

Cm

Bk

Cf

Es

Fm

Md

No

Lr

Fig. 2.1 Periodic table of the elements. Metals and some metalloids are indicated. The transition elements, the rare earth elements (lanthanide series, actinide series) and the lead-group elements on the right hand side of the table are relevant to the definition of "heavy metals" provided in this chapter

Fig. 2.1 Periodic table of the elements. Metals and some metalloids are indicated. The transition elements, the rare earth elements (lanthanide series, actinide series) and the lead-group elements on the right hand side of the table are relevant to the definition of "heavy metals" provided in this chapter metals, metalloids and nonmetals. This includes the elements of the third to seventh main groups of the periodic table, but excludes the rare gases (the eighth main group). Metallic members of this group include Al, Ga, In, Tl, Sn, Pb, Cb, Bi, Te and Po. All of them (except Bi) form amphoteric oxides. Si, Ge, As and Te are considered to be metalloids; sometimes B and Sb are included too (Fig. 2.1). Since there is no common name for the metal/metalloid members of the p-group, we suggest that these metals and metalloids should be termed "lead-group elements", as lead is the representative of this group that has been studied in the greatest depth in plant science.

As plant scientists, we should stress at this point that we never talk about the elemental forms of these elements. We usually only deal with their salts. There are, of course, special cases where the properties of a compound formed from elements from any of the groups defined above are modified (e.g. by organic ligands or sub-stituents). This should then be treated as a special case and does not necessarily have an impact on the divisions and subdivisions of elements. Classifying metals according to their positions in the periodic table of the elements makes sense because the chemical properties of their compounds are related to it.

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