Transition Elements

I

Introduction

Transition Elements, series of chemical elements that share similar electron orbital structures and hence similar chemical properties. The transition elements are commonly defined as the elements with atomic numbers 21 to 30, 39 to 48, 72 to 80, and recently elements 71 and 103 have been considered as transition metals. They occupy the groups (columns) in the periodic table numbered 3 to 12 (or, alternatively, IIIb to IIb). Transition metals include many of the metals encountered in everyday life, such as iron, copper, nickel, and gold.

II

Properties

The transition elements exhibit multiple valences, or oxidation states, typically ranging from 0 to +8 in compounds. In organometallic compounds, consisting of metals bonded to organic species, transition metals sometimes take on negative oxidation states. The transition elements have such typical metallic properties as malleability, ductility, high conductivity of heat and electricity, and metallic lustre. They tend to act as reducing agents (donors of electrons), but are less active in this regard than the alkali metals and alkaline earth metals, which have valences of +1 and +2, respectively. Transition elements in general have high densities and freezing points, and exhibit magnetic properties. They form both ionic and covalent bonds with anions (negatively charged ions), and such compounds are in general brightly coloured.

III

Electron Arrangements

The atoms of transition metals have their highest-energy electrons in d-orbitals—3d for the first transition series, 4d for the second transition series, and 5d for the third transition series. However, their outer electrons (those furthest from the nucleus) are normally in s-orbitals. The electron arrangement of iron, for example, is 1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁶ 4s². Cobalt has one more electron and has the arrangement 1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁷ 4s². Thus the outermost electron arrangement is the same and the two elements have very similar properties.

The first transition series ranges from scandium (3d⁰4s²) to zinc (3d¹⁰4s²); the added electrons go into the inner 3d shell.

IV

Colours of Transition-Metal Compounds

The colours of compounds of transition metals are explained by their electron arrangements. In most molecular environments there are small differences in energy between the five sub-levels of the d-orbitals. The energy gap between these corresponds to the energy of a photon of visible light of a definite wavelength, or colour. So transition metal compounds can absorb a photon of that colour, which promotes an electron from one d-orbital to another. This causes an alteration of colour in the light transmitted or reflected by that compound. Relatively small chemical changes can affect the energy levels of the d-orbitals and thus the colours of the compounds. Thus iron(II) hydroxide, Fe(OH)₂, is green and iron(III) hydroxide, Fe(OH)₃, is brown.