Electron Configurations and Properties of Transition metals
A concise revision guide to the transition metals: electron configurations of the d-block atoms and ions from scandium to zinc, what makes an element a transition metal, and why they show variable oxidation numbers.
- 15A.1
- 15A.2
- 15A.3
What these spec points say
- 15A.1 be able to deduce the electronic configurations of atoms and ions of the d-block elements of period 4 (Sc–Zn), given the atomic number and charge (if any)
- 15A.2 know that transition metals are d-block elements that form one or more stable ions with incompletely-filled d-orbitals
- 15A.3 understand why transition metals show variable oxidation number
Recap: Electron Configurations
Five quick questions on what you already know: the order in which sub-shells fill, why 4s fills before 3d, and writing configurations for atoms up to krypton.
The d-Block: Scandium to Zinc
The d-block elements of Period 4 run from scandium (Z = 21) to zinc (Z = 30). Across the block, electrons are added to the 3d sub-shell, which holds up to ten electrons in five orbitals.
- The 4s sub-shell fills before the 3d sub-shell.
- [Ar] stands for the argon core, 1s²2s²2p⁶3s²3p⁶.
- Either order of writing is accepted: [Ar]3d⁶4s² or [Ar]4s²3d⁶.
- Your earlier notes, Topic 1 Atomic Structure and the Periodic Table, cover the filling rules.
| Element | Atomic number | Electron configuration | 3d electrons | 4s electrons |
|---|---|---|---|---|
| scandium, Sc | 21 | [Ar]3d¹4s² | 1 | 2 |
| titanium, Ti | 22 | [Ar]3d²4s² | 2 | 2 |
| vanadium, V | 23 | [Ar]3d³4s² | 3 | 2 |
| chromium, Cr | 24 | [Ar]3d⁵4s¹ | 5 | 1 |
| manganese, Mn | 25 | [Ar]3d⁵4s² | 5 | 2 |
| iron, Fe | 26 | [Ar]3d⁶4s² | 6 | 2 |
| cobalt, Co | 27 | [Ar]3d⁷4s² | 7 | 2 |
| nickel, Ni | 28 | [Ar]3d⁸4s² | 8 | 2 |
| copper, Cu | 29 | [Ar]3d¹⁰4s¹ | 10 | 1 |
| zinc, Zn | 30 | [Ar]3d¹⁰4s² | 10 | 2 |
The ten d-block elements of Period 4 with their 3d and 4s electrons shown in boxes. Chromium and copper break the pattern.
Remember: Within the 3d sub-shell, electrons occupy the five orbitals singly before any pairing starts. Manganese, 3d⁵, has five unpaired 3d electrons. Iron, 3d⁶, has one pair and four unpaired electrons.
Chromium and Copper: the Two Exceptions
Chromium and copper each have only one 4s electron. One electron that would be expected in 4s is found in 3d instead.
| Element | Expected (wrong) | Actual | 3d arrangement |
|---|---|---|---|
| chromium | [Ar]3d⁴4s² | [Ar]3d⁵4s¹ | half-filled: five unpaired electrons |
| copper | [Ar]3d⁹4s² | [Ar]3d¹⁰4s¹ | full: five pairs |
- The 3d and 4s sub-shells are very close in energy.
- A half-filled or a full 3d sub-shell is a lower-energy, more stable arrangement.
- In chromium, all six outer electrons are unpaired, so repulsion between paired electrons is avoided.
Exam focus: Write out the configurations of chromium and copper from memory. They are the two most commonly tested atoms in the d-block, and most errors come from writing 3d⁴4s² or 3d⁹4s².
Electron Configurations of Ions
When a d-block atom forms a positive ion, the 4s electrons are lost first, before any 3d electrons. Once the 3d sub-shell holds electrons, 4s is the higher in energy.
- Write the configuration of the atom.
- Remove the 4s electrons first.
- Remove 3d electrons only if more must be lost.
Worked example: Fe²⁺ and Fe³⁺ (Z = 26)
Step 1. The atom is [Ar]3d⁶4s².
Step 2. Fe²⁺ has lost two electrons, both from 4s: [Ar]3d⁶.
Answer. Fe³⁺ has lost one more electron, from 3d: [Ar]3d⁵.
Worked example: Cu²⁺ (Z = 29)
Step 1. The atom is [Ar]3d¹⁰4s¹.
Answer. Remove the one 4s electron, then one 3d electron: Cu²⁺ is [Ar]3d⁹.
Worked example: Cr³⁺ (Z = 24)
Step 1. The atom is [Ar]3d⁵4s¹.
Answer. Remove the one 4s electron, then two 3d electrons: Cr³⁺ is [Ar]3d³.
| Ion | Configuration | Ion | Configuration |
|---|---|---|---|
| Sc³⁺ | [Ar] | Fe³⁺ | [Ar]3d⁵ |
| Ti³⁺ | [Ar]3d¹ | Co²⁺ | [Ar]3d⁷ |
| V³⁺ | [Ar]3d² | Ni²⁺ | [Ar]3d⁸ |
| Cr³⁺ | [Ar]3d³ | Cu⁺ | [Ar]3d¹⁰ |
| Mn²⁺ | [Ar]3d⁵ | Cu²⁺ | [Ar]3d⁹ |
| Fe²⁺ | [Ar]3d⁶ | Zn²⁺ | [Ar]3d¹⁰ |
Iron loses its two 4s electrons to form Fe²⁺, then one 3d electron to form Fe³⁺. The ions of scandium and zinc have an empty and a full 3d set.
Common mistake: Fe²⁺ is [Ar]3d⁶, not [Ar]3d⁴4s². No simple ion of a Period 4 d-block element keeps any 4s electrons.
Check: Configurations of Atoms and Ions
Write and recognise the electron configurations of d-block atoms and of ions that were not worked through above.
What Is a Transition metal?
Not every d-block element is a transition metal. The test is applied to the ions that the element forms.
Definition: A transition metal is a d-block element that forms one or more stable ions with incompletely filled d orbitals.
| Element | Only stable ion | Configuration of the ion | Transition metal? |
|---|---|---|---|
| scandium | Sc³⁺ | [Ar], 3d⁰: empty | no |
| zinc | Zn²⁺ | [Ar]3d¹⁰: full | no |
| iron | Fe²⁺ and Fe³⁺ | 3d⁶ and 3d⁵: incomplete | yes |
| copper | Cu⁺ and Cu²⁺ | 3d¹⁰ and 3d⁹: Cu²⁺ is incomplete | yes |
- Scandium forms only Sc³⁺, which has no 3d electrons.
- Zinc forms only Zn²⁺, which has a full 3d sub-shell.
- Copper qualifies through Cu²⁺, even though Cu⁺ is 3d¹⁰. One ion with an incomplete set is enough.
- Scandium and zinc are d-block elements but not transition metals.
Exam focus: To explain why zinc is not a transition metal, name the ion and give its configuration: zinc forms only Zn²⁺, [Ar]3d¹⁰, which has a full 3d sub-shell.
Characteristic Properties
The transition metals share four properties that set them apart from the s-block metals. All four come from the incomplete 3d sub-shell.
| Property | What it means | Notes |
|---|---|---|
| variable oxidation number | each element forms ions and compounds in more than one oxidation number | Redox Chemistry of Transition metals |
| coloured ions | their compounds and solutions are usually coloured | Colour of Transition metal Ions |
| complex formation | their ions bond to ligands to form complex ions | Ligands, Complex Ions and Their Shapes |
| catalytic activity | the elements and their compounds speed up many reactions | Heterogeneous Catalysis |
Remember: Scandium and zinc compounds are white and their solutions are colourless. Each has only one oxidation number in its compounds: +3 for scandium and +2 for zinc.
Quick Check: Which Property Is This?
Each fact shows one characteristic property. Choose it quickly.
Why Transition metals Have Variable Oxidation numbers
The 3d and 4s sub-shells are very close in energy. Different numbers of these electrons can be lost or shared without a large energy cost.
- Successive ionisation energies rise gradually, with no large jump until all the 3d and 4s electrons have gone.
- Calcium: 590, 1145, then 4912 kJ mol⁻¹. The third electron comes from an inner shell, so calcium is only ever +2.
- Vanadium: 651, 1414, 2830, 4507, then 6299 kJ mol⁻¹. The rise is steady, so +2, +3, +4 and +5 are all found.
- The extra energy needed to remove another electron is repaid by the larger lattice energy or hydration energy of the more highly charged ion.
| Element | Common oxidation numbers | Examples |
|---|---|---|
| titanium | +3, +4 | Ti³⁺, TiO₂ |
| vanadium | +2, +3, +4, +5 | V²⁺, V³⁺, VO²⁺, VO₂⁺ |
| chromium | +2, +3, +6 | Cr²⁺, Cr³⁺, Cr₂O₇²⁻ |
| manganese | +2, +4, +6, +7 | Mn²⁺, MnO₂, MnO₄²⁻, MnO₄⁻ |
| iron | +2, +3 | Fe²⁺, Fe³⁺ |
| cobalt | +2, +3 | Co²⁺, Co³⁺ |
| nickel | +2 | Ni²⁺ |
| copper | +1, +2 | Cu⁺, Cu²⁺ |
The common values for titanium to copper. The highest value rises to +7 at manganese, where all the 3d and 4s electrons are used.
- Low oxidation numbers, such as +2 and +3, are found in simple ions like Fe²⁺.
- High oxidation numbers are found in ions with oxygen, such as MnO₄⁻, where the metal is covalently bonded.
- The highest value from titanium to manganese equals the total number of 3d and 4s electrons.
Exam focus: The key phrase is that the 3d and 4s electrons have similar energies. Add that successive ionisation energies increase only gradually, so several oxidation numbers are energetically possible.
Check: Transition metals and Their Properties
Apply the definition to ions you have not met on this page, and explain why more than one oxidation number is possible.
Quick Check: Reading Ionisation Energy Data
Use new ionisation energy data to compare a transition metal with a metal that is not one.
Common Exam Points
- Chromium is [Ar]3d⁵4s¹ and copper is [Ar]3d¹⁰4s¹.
- Ions lose 4s electrons before 3d electrons: Fe²⁺ is [Ar]3d⁶ and Cu²⁺ is [Ar]3d⁹.
- Always write the atom first, then remove electrons. Do not work from the element with the same number of electrons.
- Sc³⁺ (3d⁰) and Zn²⁺ (3d¹⁰) do not have an incomplete 3d sub-shell, so scandium and zinc are not transition metals.
- The four characteristic properties are variable oxidation number, coloured ions, complex formation and catalytic activity.
- Variable oxidation numbers arise because the 3d and 4s electrons are close in energy.
FAQs
Use these quick answers to check electron configurations, the definition of a transition metal and variable oxidation numbers.
Why is chromium 3d⁵4s¹ and not 3d⁴4s²?
The 3d and 4s sub-shells are very close in energy. One electron in each of the five 3d orbitals and one in 4s avoids pairing, which lowers repulsion. Copper is the other exception: [Ar]3d¹⁰4s¹, with a full 3d sub-shell.
Which electrons are lost first when a transition metal forms an ion?
The 4s electrons are lost before the 3d electrons, even though 4s fills first. Iron is [Ar]3d⁶4s², so Fe²⁺ is [Ar]3d⁶ and Fe³⁺ is [Ar]3d⁵. Write the configuration of the atom first, then remove electrons from 4s.
Why are scandium and zinc not transition metals?
Scandium forms only Sc³⁺, which is [Ar] with an empty 3d sub-shell. Zinc forms only Zn²⁺, which is [Ar]3d¹⁰ with a full one. Neither has an ion with an incomplete 3d sub-shell, so both are d-block elements but not transition metals.
Why do transition metals have variable oxidation numbers?
The 3d and 4s sub-shells are close in energy, so successive ionisation energies rise only gradually. The extra energy needed to remove another electron is offset by the energy released when the ion forms bonds or is hydrated.
What are the characteristic properties of the transition metals?
They show variable oxidation numbers, form coloured ions, form complex ions with ligands and act as catalysts. All four properties arise from the incomplete 3d sub-shell. Copper qualifies through Cu²⁺, which is [Ar]3d⁹.
Copyright notice: This OLS revision content, including the explanations, layout, diagrams, tables and embedded learning structure, is authored for Online Learning System by Dr. Mohammed Al-Fatah. It may not be copied, reproduced, redistributed or adapted without written permission.
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