Transition Metals

FIELDS OF STUDY: Inorganic Chemistry; Geochemistry; Metallurgy

ABSTRACT

The basic properties and characteristics of the transition metals are presented. The transition metals are those elements whose valences are determined by the behavior of electrons in their d orbitals.

The Nature of the Transition Metals

The transition metals consist of the various elements in the d-block of the periodic table. The valence electrons of a transition metal atom are located in the atom’s d orbitals, specific regions around the atom’s nucleus. This arrangement of electrons is responsible for the multivalent nature of transition metal ions and for their ability to form numerous compounds with neutral organic molecules. The number of electrons allowed to occupy the d orbitals enables transition metal atoms to form compounds in which they take on different oxidation states. Many of the transition metals, including titanium, iron, gold, and mercury, are readily recognized and well known. Others, such as osmium and iridium, are scarce and thus much less familiar.

Atomic Structure of the Transition Metals

As with all elements, the atomic structure of transition metals consists of a central nucleus, which comprises positively charged protons and neutral neutrons, and negatively charged electrons, which are typically described as filling regions around the nucleus known as "orbitals." Types of orbitals include s orbitals, p orbitals, and d orbitals. The d orbitals first become relevant in atomic structure in the elements of the fourth period, or row, of the periodic table. Scandium is the first fourth-period element to be able to have electrons in its d orbital. Electrons are permitted to shift from orbital to orbital in response to environmental influences such as the types and numbers of ions or molecules that surround the particular atom in a compound. The transition metals are therefore readily able to take part in reduction-oxidation (redox) reactions, in which the oxidation states of the atoms involved change, due to the ease with which they donate or accept different numbers of valence electrons.

Transition Metal Compounds

The transition metals commonly form both monatomic and complex ions as well as coordination compounds in which a central transition metal atom is surrounded by a specific number of ligands, molecules or atoms that are often neutral or anionic. The bonds to the surrounding ligands are termed "coordination bonds" and are thought to form when an electron pair from the ligand enters a vacant orbital on the metal atom. Due to the strictly defined geometry of atomic orbitals, the ligands about the central metal atom can take on only select spatial arrangements, coordinating in structures that include the trigonal pyramid and trigonal planar arrangements of three ligands, the square planar and tetrahedral arrangements of four ligands, the trigonal bipyramid and square pyramid arrangements of five ligands, and the square bipyramid arrangement of six ligands. Higher coordination complexes are also known but are much less common. In addition, the same ligands can be shared by two metal atoms and thus form an extended structure, as in the chromate (CrO₄²⁻) and dichromate (Cr₂O₇²⁻) ions.

Transition Metals and Nomenclature

Historically, the study of metallurgy in medieval times marked the beginning of the practical science of chemistry. Because of the field’s historical roots, many of the chemical symbols for elements derive from the medieval Latin names for the materials. In addition to the Latin root word, the name of a transition metal compound can reveal a great deal of information about the substance, including its oxidation state. For example, Pb(NO₃)₂ is lead (II) nitrate. The (II) indicates that the lead atom is in the +2 oxidation state. The chemical symbol for lead is Pb because it derives from plumbum, the Latin word for the element.

Extraction of Transition Metals

The transition metals are normally found in nature as ores or compounds and are typically extracted by mining and smelting operations that release the metals as relatively pure elements. It is also possible to find some of the transition metals as native metals—metals that exist in nature in their metallic form—rather than as ores or mineral compounds, but this is a rarity and requires very specific environmental conditions. Some of the transition metals, such as gold and platinum, are normally inert and are always found in their elemental form in nature rather than as compounds.

PRINCIPAL TERMS

• d-block: the portion of the periodic table containing the elements whose valence electrons are in their d orbitals.
• element: a form of matter consisting only of atoms of the same atomic number.
• oxidation state: a number that indicates the degree to which an atom or ion in a chemical compound has been oxidized or reduced.
• periodic table: the chart representing the known elements by atomic number and electron distribution.
• valence electron: an electron that occupies the outermost or valence shell of an atom and participates in chemical processes such as bond formation and ionization.

Bibliography

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