First Row Transition Elements: Electronic Configuration and Properties: PECTAA Class 12 Chemistry, Punjab Board

Iron rusts to a reddish-brown compound, copper sulphate crystals are blue and potassium manganate(VII) is deep purple. Colour, magnetism, catalysis and many oxidation states all trace back to one feature of these metals: a partly filled 3d subshell.

Chapter 18 of the PECTAA Chemistry 12 book is Transition Metals. Chapter 17 (Group 2 elements) ended with the uses of Group 2 compounds. Section 18.1 introduces the first row (3d) transition elements and their electronic configurations, and section 18.2 lists their physical and chemical properties. This note is for FSc Part 2 (2nd year, Class 12) students in Lahore, Faisalabad, Sargodha, Rawalpindi, Multan, Gujranwala and the other Punjab boards. Gujrat district comes under the Gujranwala board. PECTAA writes the books. Your own board sets your paper.

What to know

Definition (18.1). First row transition elements lie in the d block and have partially filled d orbitals in any of their stable oxidation states. Their name reflects the transitions they show between variable oxidation states. The d block runs from group 3 to group 12, but the book does not count scandium and zinc as transition elements. Zinc forms only Zn²⁺, which is [Ar] 3d¹⁰ 4s⁰: a complete 3d subshell. The book's chapter introduction also notes that transition metals divide into d-block and f-block metals, and the chapter focuses on the 3d series.

Configurations (18.1.1, Table 18.1). The order usually follows the Aufbau principle, except for chromium and copper, which gain stability from a half-filled (3d⁵) or completely filled (3d¹⁰) subshell. When ions form, the 4s electrons go first.

(All ion configurations are [Ar] 3dⁿ 4s⁰.)

Physical properties (18.2.1).

• Hard, rigid solids.
• Silvery or grey and lustrous, except reddish-brown copper.
• Atomic radius usually decreases from left to right, with a small rise at the end, linked to copper's filled d subshell.
• High melting and boiling points, with a variable trend across the row (Fig. 18.1).
• Good conductors of heat and electricity.
• High densities that generally increase from titanium to copper, because nuclear charge builds up and atomic size shrinks.
• Shiny, malleable (hammered into sheets) and ductile (drawn into wires).
• Most of the metals and their compounds are paramagnetic because of unpaired d electrons.

Chemical properties (18.2.2).

• Variable oxidation states, e.g. Fe²⁺ and Fe³⁺.
• A strong tendency to form coordination compounds (complexes): vacant d orbitals accept lone pairs from ligands.
• Coloured ions and complexes, because electrons move from a lower to a higher energy level when light is absorbed.
• Catalytic behaviour, because they change oxidation state easily.

Key points

• Partially filled d subshell in a stable oxidation state = transition element.
• Cr 3d⁵ 4s¹ and Cu 3d¹⁰ 4s¹ are the exceptions.
• Ions lose 4s electrons before 3d.
• Zn²⁺ is 3d¹⁰, so zinc is excluded.

Worked examples

Example 1. Write the configuration of Mn²⁺.

1. Mn (Z = 25) is [Ar] 3d⁵ 4s².
2. Remove the two 4s electrons: [Ar] 3d⁵ 4s⁰.

Example 2. Why is Fe³⁺ often described as stable?

1. Fe is [Ar] 3d⁶ 4s². Fe³⁺ loses both 4s electrons and one 3d electron.
2. Fe³⁺ = [Ar] 3d⁵: a half-filled 3d subshell, the same extra stability the book uses to explain chromium.

Example 3. Count the unpaired electrons in Cu²⁺ and predict its magnetic behaviour.

1. Cu²⁺ = [Ar] 3d⁹.
2. Nine electrons in five d orbitals: four pairs and one unpaired electron.
3. One unpaired electron, so Cu²⁺ is paramagnetic.

Example 4. Is copper a transition element even though Cu is 3d¹⁰?

1. The test is any stable oxidation state.
2. Cu²⁺ is 3d⁹, a partially filled subshell, so yes.

Example 5 (reading Fig. 18.1). The book's graph gives atomic radii (pm) of 147 (Ti), 134 (V), 128 (Cr), 127 (Mn), 126 (Fe), 125 (Co), 124 (Ni) and 128 (Cu). Describe the trend.

1. From Ti to Ni the radius falls, quickly at first and then only slightly.
2. At Cu it rises again, from 124 to 128 pm.
3. The book links this small rise to copper's completely filled d subshell.

Common mistakes in the exam

1. Removing 3d electrons before 4s when writing ions.
2. Writing Cr as 3d⁴ 4s² or Cu as 3d⁹ 4s².
3. Calling zinc a transition element because it is in the d block.
4. Linking colour to the metal alone: the book links it to electron transitions when light is absorbed.
5. Forgetting that paramagnetism needs unpaired electrons.

Exam technique

• For any ion: write the atom first, then remove 4s electrons, then 3d.
• Draw d-orbital boxes when counting unpaired electrons.
• For "why" questions, give the reason in one line (half-filled or filled stability; vacant d orbitals; easy change of oxidation state).
• Check your board's notice for the sections in your paper.

Practice questions

1. Configuration of V³⁺? Answer: [Ar] 3d² 4s⁰.
2. Configuration of Co²⁺? Answer: [Ar] 3d⁷ 4s⁰.
3. Which first-row metal is reddish brown? Answer: Copper.
4. How many unpaired electrons does Ti³⁺ have? Answer: One (3d¹).
5. Why do transition metals act as catalysts? Answer: They can change their oxidation states easily.

Ready to practise this? Continue in 2nd Year Chemistry.