Transition elements and oxidation states (HL)
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Question
What is a transition element?
Answer
A **d-block metal** that forms **at least one stable ion with a partially filled d sub-shell**.
Question
Why are Sc and Zn often excluded?
Answer
Their only ions are **Sc³⁺ ([Ar] 3d⁰)** and **Zn²⁺ ([Ar] 3d¹⁰)** — empty/full d, never **partially filled**.
Question
Which sub-shell fills first, 4s or 3d?
Answer
**4s fills first** (slightly lower energy when empty), so atoms end in **3d^{x} 4s²**.
Question
How do you write a transition-metal ion?
Answer
**Remove 4s electrons before 3d.** e.g. Fe²⁺ = [Ar] 3d⁶ (the two 4s electrons go first).
Question
Electron configuration of chromium?
Answer
**[Ar] 3d⁵ 4s¹** — an anomaly; a **half-full 3d⁵** is extra stable.
Question
Electron configuration of copper?
Answer
**[Ar] 3d¹⁰ 4s¹** — an anomaly; a **full 3d¹⁰** is extra stable.
Question
Why do transition metals show variable oxidation states?
Answer
The **4s and 3d sub-shells are close in energy**, so electrons can be removed in steps for similar energies → several stable states.
Question
Common oxidation states of iron?
Answer
**+2 and +3** (Fe²⁺ = [Ar] 3d⁶; Fe³⁺ = [Ar] 3d⁵, a stable half-full sub-shell).
Question
Common oxidation states of copper?
Answer
**+1 and +2** (Cu⁺ in Cu_{2}O, Cu²⁺ in CuSO_{4}).
Question
Oxidation state of Mn in MnO_{4}⁻?
Answer
**+7** — four O at −2 (−8) with an overall −1 charge forces Mn to +7.
Question
Why are many transition-metal compounds coloured?
Answer
The **partially filled d sub-shell** splits in a ligand field and **absorbs visible light**.
Question
Why are transition metals good catalysts?
Answer
They can **change oxidation state** and use empty/part-full **d orbitals** to bind reactants (e.g. Fe in the Haber process).
Question
What makes a transition-metal compound paramagnetic?
Answer
Having **unpaired d electrons** — these are drawn into a magnetic field.
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Full study notes for Transition elements and oxidation states (HL)
Topic 3.1 hub
The periodic table: classification of elements
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