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The trace elements of Group 4 are titanium (Ti), zirconium (Zr), and hafnium (Hf). Titanium is considered a trace element only because of its low concentrations in plant and animal tissues. The physical and chemical properties of these metals differ (Table II-4.1), but they all are widely distributed in both litho- and biosphere. However, their biochemical functions are hardly known.

Geochemical characteristics of the metals of Group 5, vanadium (V), niobium (Nb) and tantalum (Ta), are variable, although all elements exhibit mainly lithophilic tendencies. Vanadium is widely distributed in geochemical environments where it forms both cationic and anionic compounds. Two elements, niobium and tantalum have very similar atomic radius and resemble each other in geochemical properties (Table II-5.1). Both metals are rather rare in the biosphere (Table II-5.2), are highly associated with Fe, Ti, Mn and Zr, and are likely to be concentrated in polymetallic concretions in the ocean bottom.

The trace metals of the Group 6, chromium (Cr), molybdenum (Mo), and tungsten (W), have strong lithophilic tendencies. They have variable oxidation states, possess bimodal redox behavior in terrestrial and aquatic environments, and are likely to be present in two oxidation states, +3 and +6 (Table II-6.1). Metals of this group have the propensity to form oxyanions and coordinate only weakly with other common environmentally ligands such as Cl or OH.

The elements of Group 7 are manganese (Mn), technetium (Tc) and rhenium (Re). Only Mn is essential to living organisms. Technetium is a by-product of nuclear fuel processing and occurs as unstable radionuclides with variable but mainly short half-lives (from seconds to years). Rhenium is highly dispersed in the environment. All elements of this group have variable valences (Table II-7.1) and exhibit both lithophile and chalcophile characters.

According to the previous nomenclature system of the Periodic Tables, the elements of the three groups of the Table: 8, 9, and 10 were included in one VIII group. The elements of the first subgroup (triad) were the so-called: iron family metals (Fe, Co and Ni). Two next triads contained noble metals of the platinum-group (Ru, Rh, Pd, Os, Ir, Pt). The old system referred to chemical and geochemical properties of the elements and therefore it was easier to compare and describe these elements. However, the new nomenclature system, as obligatory, will be followed in the text.

The elements of Group 9 are cobalt (Co), rhodium (Rh), and iridium (Ir). Cobalt belongs to the iron family of metals that include also Fe and Ni, whereas Rh and Ir are noble metals of the platinum group (PGMs).

Group 10 contains three elements: nickel (Ni), palladium (Pd), and platinum (Pt). Nickel belongs to the iron family that also includes Fe and Co, whereas Pd and Pt are noble metals of the PGMs.

The Group 11 is composed of copper (Cu), silver (Ag), and gold (Au). Silver and Au belong to the so-called noble metals. All these metals occur in variable oxidation stages, mainly +1, and +2, (Table II-11.1). Copper and Ag reveal chalcophilic tendencies whereas Au is siderophilic.

The Group 12 consists of zinc (Zn), cadmium (Cd), and mercury (Hg). These metals have quite a low abundance in the Earth’s crust. These metals form compounds in which their oxidation states are usually not higher than +2 and easily form metal-metal (M-M) bonds (Table II-12.1). The strength of the bond increases down the group, in the following order: Hg < Cd < Zn. The Zn and Cd ions are highly unstable, however, the +1 state of Hg is quite stable compared with the other two elements. The toxicity of Cd and Hg is well known, whereas Zn has enormous biological importance.

The Group 13 consists of five elements: boron (B), aluminum (Al), gallium (Ga), indium (In), and thallium (TI) (Table II-13.1). These elements are characterized by having three electrons in their outer energy levels, but reflect a wide range in the occurrence and behavior. The geochemistry of these widely distributed elements is quite complicated. Among them, only B is metalloid and plays a significant role in plants. Aluminum, being one of the basic constituents of the lithosphere, reveals amphoteric properties. Three other trace metals are widely distributed in the lithosphere and, due to their special properties, share an important position in the field of electronic industry.