Transition Metals

d-Block Elements Between groups 2 and 3 in the periodic table are found the d-block elements. You may recall that in d-block elements, electrons are being added to the d subshell of the third and subsequent shells. The first row of d-block elements consists of those elements whose highest energy electrons are filling the 3d subshell.

d-Block Elements These elements are scandium, titanium, vanadium, chromium, manganese, iron, cobalt, nickel, copper and zinc. Of these, the elements from titanium to copper are considered to be transition elements. ▫This distinction is due to the definition of a transition element as a d-block element that can form at least one stable ion with a partially filled d subshell.

d-Block Elements If we consider the electron configuration of these d-block elements and the ions that they can form, we can see clearly why scandium and zinc are not included as transition metals.

d-Block Elements

In the case of scandium and zinc, only one ion exists and that ion does not have a partially filled d subshell. ▫The scandium(III) ion, Sc 3+, is the only stable ion of scandium ad has no electrons in the 3d subshell. ▫The zinc(II), Zn 2+, ion is the only stable ion of zinc to exist and this ion has a full 3d subshell.

Transition Elements Properties of transition elements: ▫Their ions can exist in variable oxidation states. ▫They have higher melting points and are harder and denser than group 1 and 2 metals. ▫A number of the elements and their compounds have catalytic properties. ▫They can form complex ions. ▫The majority of their complexes are coloured.

Variable Oxidation States Transition elements can form ions with a variety of oxidation states (oxidation numbers). All transition elements can form ions with an oxidation of +2, and, in addition, each element can form a number of ions with other oxidation numbers.

Variable Oxidation States From titanium to manganese, the maximum oxidation number possible is equal to the total number of 4s and 3d electrons. For example, the maximum oxidation number possible for titanium ([Ar]3d 2 4s 2 ) is +4 and for manganese ([Ar]3d 5 4s 2 ) it is +7.

Variable Oxidation States

The transition elements form ions by losing electrons from both the 4s and the 3d subshells. This is possible because these subshells are very close to each other in energy.

Variable Oxidation States The variation in oxidation states is easily recognizable in many transition elements by a change in colour in the new compound.

Physical Properties The d-block elements are all metals. They have physical properties that are typical of metals. ▫With the exception of mercury (liquid at room temperature), their melting points are high and they are solids under standard conditions. ▫They are good conductors of electricity and heat. ▫They are hard, strong and shiny.

Physical Properties The chemical reactivity of the d-block elements is relatively low and, together with these physical properties, makes d- block elements extremely useful. Iron is used widely for construction of bridges, buildings, vehicles and other structures that require great strength. Copper is most valuable for its excellent conduction of electricity and as unreactive, yet malleable, metal for pipes.

Catalytic Properties A catalyst increases the rate of a chemical reaction by providing an alternative reaction pathway with a lower activation energy.

Catalytic Properties The catalyst itself is not consumed. As the activation energy is lowered, there will be a greater proportion of particles present at a given temperature with sufficient kinetic energy to overcome the activation energy.

Catalytic Properties Many transition metals and their compounds show catalytic activity and they are widely used in industry (where it is desirable to generate maximum quantities of product as quickly as possible to maximize profits).

Catalytic Properties Solid catalysts have a high surface energy (the amount of energy required to create a new surface). Due to their high surface energies, solid catalysts are able to form strong bonds with the molecules that come into contact with them.

Catalytic Properties This attraction at the surface of the catalyst weakens the bonds within the reactant molecules, making it easier to break them apart. Collisions with other molecules are now more likely to overcome the activation energy for the reaction, and so the rate of the product formation is increased.

Catalytic Properties Nickel is used as a catalyst in the hydrogenation of alkenes to form alkanes. A practical application of this process is in the manufacture of margarine. The vegetable oils used for making margarines are refined as liquids that are high in unsaturated fatty acids. To increase the melting point of the mixture, the fatty acids undergo hydrogenation during which some carbon-carbon double bonds are converted to single bonds and hydrogen is added.

Catalytic Properties Platinum, rhodium and palladium are particularly effective industrial catalysts. Alloys of platinum and rhodium are used in the catalytic converters of car exhaust systems, where toxic gaseous emissions such as CO, NO and NO 2 are broken down into harmless compounds.

Catalytic Properties Biological catalysts are better known as enzymes. Hydrogen peroxide is a toxin in the human body. Its decomposition is catalyzed by the enzyme catalase. In the lab the decomposition of hydrogen peroxide can be catalyzed by a number of inorganic catalysts, such as MnO 2.