Transition metal properties: Cr to Cu
Atomic structure and the periodic table • Properties of transition metals
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Definition and limiting factors
A transition metal is an element that forms at least one ion with a partially filled d subshell. This definition excludes elements that only form ions with empty or full d subshells, for example scandium in the Sc3+ state (empty d subshell) and zinc in the Zn2+ state (full d subshell), so those elements are not transition metals by this definition. The presence of partially filled d orbitals dictates the common chemical behaviour that distinguishes transition metals from s- and p-block elements.
Variable oxidation states: cause and examples
Partially filled d orbitals allow loss or sharing of different numbers of electrons, so transition metals show several stable oxidation states. Chromium commonly shows +2, +3 and +6 oxidation states; manganese shows +2 up to +7; iron shows +2 and +3 predominately; cobalt shows +2 and +3; nickel commonly shows +2; copper shows +1 and +2. Variable oxidation states cause rich redox chemistry and underpin catalytic cycles, because an element changes oxidation state to accept or donate electrons during reactions.
Coloured ions and compounds: electronic transitions
Partially filled d orbitals permit electronic transitions between split d levels when ligands create a crystal-field splitting. Light absorption corresponding to these transitions causes visible colours in many complex ions. For example, aqueous Cu2+ appears blue due to d–d transitions; Fe2+ and Fe3+ produce pale green and yellow-brown colours respectively; Co2+ typically produces pink or blue colours depending on ligand; MnO4− (permanganate) is intensely purple because of charge-transfer and d–d transitions at high oxidation state.
Catalytic activity: mechanism and examples
Variable oxidation states and available d orbitals provide sites for substrate adsorption and electron transfer, so transition metals act as catalysts. Iron atoms in the Haber process cycle between oxidation states to activate nitrogen, providing electrons to break the N≡N bond. Manganese and copper compounds act as oxidation catalysts in organic and inorganic reactions by facilitating electron transfer. The surface atoms of nickel and cobalt catalyse hydrogenation reactions because adsorption weakens bonds in reactant molecules.
Physical properties: bonding and consequences
Metallic bonding involving delocalised d and s electrons causes high melting points, high density, electrical conductivity and malleability. Iron, cobalt and nickel show strong metallic bonding that leads to high melting points and good electrical conductivity. Copper exhibits excellent electrical conductivity because conduction electrons move freely through overlapping d and s orbitals. Alloys form easily because atoms of transition metals have similar atomic radii and bonding, so mixing alters strength and corrosion resistance.
Magnetism and alloys: cause and effects
Unpaired d electrons cause paramagnetism or ferromagnetism. Iron, cobalt and nickel exhibit ferromagnetism at room temperature because unpaired electrons align in domains, producing strong permanent magnetism. Chromium and manganese show antiferromagnetic or complex magnetic ordering depending on crystal structure. Alloy formation with iron (e.g., steels with nickel or chromium) changes mechanical properties and corrosion resistance because added atoms distort the metallic lattice and alter electron interactions.
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