Chapter 6: Transition Elements – Chemistry 2nd Year Notes

This chapter covers the Transition Elements from the 2nd Year (FSc Part-II) Chemistry syllabus of the Punjab Curriculum and Textbook Board (PTB/PCTB). Transition elements are the d-block metals in the middle of the periodic table, defined as elements whose atoms or common ions have partially filled d-orbitals. The first transition series runs from scandium to zinc.

Their special properties, variable oxidation states, coloured ions, catalytic activity, magnetic behaviour and the formation of complex compounds, all come from their partly filled d-orbitals.

Learning Objectives

• Define transition elements and give the first transition series.
• Explain why transition metals show variable oxidation states.
• Explain why their ions are coloured and many are paramagnetic.
• Explain their catalytic activity and the formation of complex ions.
• Define ligand, coordination number and complex, with examples.
• State important uses and alloys of transition metals.

Key Concepts

What Makes an Element a Transition Element

A transition element is one whose atom or a stable ion has an incompletely filled d-subshell. On this strict definition zinc is not a typical transition metal because its ion Zn2+ has a completely filled 3d10 configuration. The first (3d) series is scandium, titanium, vanadium, chromium, manganese, iron, cobalt, nickel, copper and zinc. Chromium and copper have the anomalous configurations 3d5 4s1 and 3d10 4s1 because half-filled and filled d subshells are extra stable.

General Characteristics

Transition metals are hard, strong, high-melting metals with high densities. Because the 3d and 4s energy levels are very close, a variable number of electrons can be used in bonding, so most show several oxidation states (iron as +2 and +3; manganese from +2 up to +7). Most of their ions are coloured because electrons jump between split d-orbitals (d to d transitions), absorbing part of visible light; for example Cu2+ is blue and the permanganate ion is purple. Ions with unpaired d-electrons are paramagnetic, and the metals and their compounds are excellent catalysts.

Complex (Coordination) Compounds

A special feature of transition metals is the formation of complex ions, in which a central metal ion is surrounded by molecules or ions called ligands, each donating a lone pair to the metal by a coordinate bond. The number of ligand atoms directly bonded to the metal is the coordination number, most often 4 or 6. Examples include the deep blue tetraamminecopper(II) ion and hexacyanoferrate ions.

Alloys and Uses

Iron is turned into steel and stainless steel (with chromium and nickel); copper is used in wiring and, with zinc, forms brass; chromium and nickel are used for electroplating; and silver, gold and platinum are prized in jewellery, electronics and as catalysts. Common catalysts include iron in the Haber process, vanadium(V) oxide in the Contact process and nickel in hydrogenation.

Important Definitions

Formulas & Rules

Item	Fact
First transition series	Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn
Anomalous configs	Cr = 3d5 4s1, Cu = 3d10 4s1
Colour cause	d to d electronic transitions
Key catalysts	Fe (Haber), V2O5 (Contact), Ni (hydrogenation)
Coordination numbers	commonly 4 and 6

Diagrams & Illustrations

The d-block: a periodic-table schematic showing the d-block transition elements between the s-block and p-block, with the first series scandium to zinc labelled.

Coloured ions: a chart of characteristic colours (Cu2+ blue, Fe2+ pale green, Fe3+ yellow-brown, permanganate purple, Cr3+ green) arising from d to d transitions.

Octahedral complex ion: a diagram of a central metal ion surrounded by six ligands (coordination number 6), each donating a lone pair.

Solved Examples & Numericals

Example 1: Variable oxidation states

Give two oxidation states of iron with a compound of each. Iron shows +2 (as in FeSO4) and +3 (as in FeCl3), possible because the 3d and 4s electrons have similar energies.

Example 2: Identifying a complex

In the tetraamminecopper(II) ion, name the central ion, ligand and coordination number. Central ion Cu2+, ligand ammonia, coordination number 4.

Example 3: Colour

Why is Zn2+ colourless while Cu2+ is blue? Zn2+ has a completely filled 3d10 shell, so no d to d transition is possible; Cu2+ has a partly filled d-shell, so it absorbs light and appears blue.

Short Questions & Answers

Long Questions & Answers

Q1: Describe the general characteristic properties of the transition elements.

The transition elements are d-block metals with partially filled d-orbitals, and this single feature explains their special properties. They are hard, dense, high-melting metals because both their 3d and 4s electrons take part in strong metallic bonding. Because these two energy levels are very close, a variable number of electrons can be used in bonding, so the metals show several oxidation states. Their ions are usually coloured because electrons move between split d-orbitals (d to d transitions) and absorb part of visible light. Ions with unpaired d-electrons are paramagnetic. The metals and their compounds are excellent catalysts, and their small, highly charged ions readily accept lone pairs from ligands to form stable complex compounds.

Q2: What are complex compounds? Explain with reference to ligands and coordination number.

A complex compound contains a complex ion, in which a central transition-metal ion is bonded to a definite number of surrounding molecules or ions called ligands. Each ligand has at least one lone pair, which it donates to the metal ion to form a coordinate bond; common ligands are water, ammonia, chloride and cyanide ions. The number of ligand atoms directly attached is the coordination number, most often 4 (tetrahedral or square-planar) or 6 (octahedral). For example, in a hexaamminecopper(II) ion the central Cu2+ is surrounded by six ammonia ligands, giving a coordination number of 6. Transition metals form such complexes readily because their small, highly charged ions with empty d-orbitals accept ligand lone pairs.

Q3: Explain the catalytic activity and important uses of transition metals.

Transition metals and their compounds are among the best catalysts because they can exist in more than one oxidation state, providing an easy path for electron transfer, and because reactant molecules can be adsorbed and activated on their surfaces. Familiar examples are iron in the Haber process, vanadium(V) oxide in the Contact process, nickel in the hydrogenation of vegetable oils, and platinum in catalytic converters. Beyond catalysis, transition metals are indispensable materials: iron and steel build machinery, stainless steel resists rust, copper carries electricity and forms brass and bronze, and chromium and nickel are used for protective electroplating.

MCQs with Answers

Quick Revision Summary

• Transition metals = d-block with partly filled d-orbitals; first series Sc to Zn.
• Zinc is not a typical transition metal (3d10).
• Key properties: variable oxidation states, coloured ions, paramagnetism, catalysis, complexes.
• Colour comes from d to d transitions; Zn2+ is colourless.
• Complex = central metal plus ligands; coordination number = ligand atoms bonded.
• Catalysts: Fe (Haber), V2O5 (Contact), Ni (hydrogenation).

Exam Tips

• Link every transition property back to partly filled d-orbitals.
• Remember the Cr and Cu configuration exceptions.
• For colour questions always mention d to d transitions; Zn2+ and Sc3+ are colourless.
• In any complex, state central metal, ligand and coordination number.
• Know the standard catalysts and the process each is used in.