4.7 - Transition Elements

Position and General Physical Properties

• Transition elements occupy Groups 3–12, between Groups 2 and 13. Examples include chromium, Cr; manganese, Mn; iron, Fe; and copper, Cu.
• They are metals that are solid and shiny, hard compared with Group 1 and 2 metals, dense, malleable and ductile, with high melting and boiling points.
• Scandium and zinc are not considered transition elements because they do not show the characteristic properties.

Special Characteristics

1. Catalytic action

• A transition element or its compound can increase reaction rate without undergoing permanent chemical change.
• Iron catalyses ammonia production in the Haber Process:

2. Coloured ions or compounds

Ion	Solution colour
$\mathrm{C}{\mathrm{r}}^{3+}$	Green
$\mathrm{C}{\mathrm{r}}_2{\mathrm{O}}_7^{2-}$	Orange
$\mathrm{M}{\mathrm{n}}^{2+}$	Pink
$\mathrm{Mn}{\mathrm{O}}_4^{-}$	Purple
$\mathrm{F}{\mathrm{e}}^{2+}$	Green
$\mathrm{F}{\mathrm{e}}^{3+}$	Brown
$\mathrm{C}{\mathrm{u}}^{2+}$	Blue

3. More than one oxidation number

Element	Examples
Chromium	$+3$ in $\mathrm{CrC}{\mathrm{l}}_3$; $+6$ in ${\mathrm{K}}_2\mathrm{C}{\mathrm{r}}_2{\mathrm{O}}_7$
Manganese	$+2$ in $\mathrm{MnC}{\mathrm{l}}_2$; $+4$ in $\mathrm{Mn}{\mathrm{O}}_2$; $+7$ in $\mathrm{KMn}{\mathrm{O}}_4$
Iron	$+2$ in $\mathrm{FeS}{\mathrm{O}}_4$; $+3$ in $\mathrm{FeC}{\mathrm{l}}_3$
Copper	$+1$ in $\mathrm{C}{\mathrm{u}}_2\mathrm{O}$; $+2$ in $\mathrm{CuO}$

4. Complex-ion formation

Complex ion	Formula
Tetraaminecopper(II)	$[\mathrm{Cu}(\mathrm{N}{\mathrm{H}}_3{)}_4{]}^{2+}$
Hexacyanoferrate(III)	$[\mathrm{Fe}(\mathrm{CN}{)}_6{]}^{3-}$
Hexacyanoferrate(II)	$[\mathrm{Fe}(\mathrm{CN}{)}_6{]}^{4-}$
Hexaaquaferrate(II)	$[\mathrm{Fe}({\mathrm{H}}_2\mathrm{O}{)}_6{]}^{2+}$

Shape-memory Alloy

• Nitinol, an alloy of nickel and titanium, returns to its original shape after bending. It is used in spectacle frames and treatment of broken bones.

Industrial Uses

Transition element or compound	Use
Iron, Fe	Haber Process catalyst for ammonia; bridge construction
Platinum, Pt	Ostwald Process catalyst for nitric acid
Vanadium(V) oxide, ${\mathrm{V}}_2{\mathrm{O}}_5$	Contact Process catalyst for sulphuric acid
Nickel, Ni or platinum, Pt	Hydrogenation of vegetable oils to make margarine
Manganese, Mn	Coloured window glass
Titanium, Ti	Paint manufacture

• Chromium gives ruby its colour; iron and titanium occur in sapphire; manganese occurs in amethyst.