Transition (d-Block) Elements — Part 2: Oxidation states, colour, magnetism and other properties: visual lesson, ISC Class 12 Chemistry

Lesson

Characteristic properties

Part 2 — the properties that make transition metals special.

In this lesson you will

Explain why transition metals show variable oxidation states and identify common states in the 3d series
Relate the colour of transition metal ions to d–d transitions
Calculate spin-only magnetic moments
Explain catalytic activity, complex formation, interstitial compounds and alloy formation

Concept

Variable oxidation states

Because the (n−1)d and ns electrons have similar energies, transition elements show several oxidation states, often differing by one. In the 3d series the maximum rises to +7 at Mn (= number of 3d + 4s electrons), then falls. Higher states occur in oxides, fluorides and oxoanions (MnO₄⁻, Cr₂O₇²⁻); low states in complexes with π-acceptor ligands such as CO (Ni(CO)₄, oxidation state 0).

Table of common oxidation states of the 3d elements
Element	Common oxidation states	Most common
Sc	+3	+3
Ti	+2, +3, +4	+4
V	+2 to +5	+5, +4
Cr	+2, +3, +6	+3
Mn	+2 to +7	+2, +4, +7
Fe	+2, +3	+3
Co	+2, +3	+2
Ni	+2	+2
Cu	+1, +2	+2
Zn	+2	+2

Worked example

Worked example: stability of oxidation states

Problem. Explain (a) why Fe³⁺ is more stable than Fe²⁺, (b) why Mn²⁺ is more stable than Mn³⁺, and (c) why Sc shows only +3.

(a) Fe³⁺ is 3d⁵ (half-filled, extra stable); Fe²⁺ (3d⁶) is easily oxidised to it.

(b) Mn²⁺ is 3d⁵; Mn³⁺ (3d⁴) readily gains an electron to reach d⁵, so it is a strong oxidising agent ( $E^\circ_{\ce{Mn^3+/Mn^2+}} = +1.57\ \text{V}$ ).

(c) Sc [Ar]3d¹4s² loses all three outer electrons to reach the noble-gas configuration of Sc³⁺.

Table of ions and the configurations that explain their stability
Ion	Configuration	Stability
Fe³⁺	d⁵	More stable than Fe²⁺
Mn²⁺	d⁵	More stable than Mn³⁺
Sc³⁺	d⁰ (noble gas)	Only stable state

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