Properties of transition metalsEdexcel A-Level Chemistry: Revision notes
Section 1
d-block elements and transition metals
The d-block elements are those in which the highest-energy electron enters a d-subshell. In period 4 these run from scandium (Z = 21) to zinc (Z = 30).
A transition metal is a d-block element that forms one or more stable ions with an incomplete d-subshell. Not every d-block element qualifies:
- Scandium forms only Sc³⁺, which is [Ar] with no d electrons.
- Zinc forms only Zn²⁺, which is [Ar]3d¹⁰ with a full d-subshell.
So scandium and zinc are d-block elements but not transition metals. Copper is a transition metal because it forms Cu²⁺, [Ar]3d⁹, even though Cu⁺ is 3d¹⁰.
Saying every d-block element is a transition metal. Scandium and zinc are not.
Section 2
Electronic configurations of atoms
Subshells fill in order of energy, and the 4s fills before 3d. The period 4 d-block atoms are:
Sc [Ar]3d¹4s²; Ti [Ar]3d²4s²; V [Ar]3d³4s²; Cr [Ar]3d⁵4s¹; Mn [Ar]3d⁵4s²; Fe [Ar]3d⁶4s²; Co [Ar]3d⁷4s²; Ni [Ar]3d⁸4s²; Cu [Ar]3d¹⁰4s¹; Zn [Ar]3d¹⁰4s².
Chromium and copper are exceptions: moving one 4s electron into 3d gives a half-full (3d⁵) or full (3d¹⁰) subshell, which is a more stable arrangement. [Ar] stands for 1s²2s²2p⁶3s²3p⁶.
Write the 3d before the 4s: [Ar]3d⁶4s² is the usual order, though 4s²3d⁶ is also accepted.
Section 3
Electronic configurations of ions
When d-block atoms form positive ions, the 4s electrons are lost first, then 3d electrons.
- Fe²⁺: [Ar]3d⁶; Fe³⁺: [Ar]3d⁵
- Cu⁺: [Ar]3d¹⁰; Cu²⁺: [Ar]3d⁹
- Cr³⁺: [Ar]3d³; Mn²⁺: [Ar]3d⁵; V³⁺: [Ar]3d²
- Zn²⁺: [Ar]3d¹⁰; Sc³⁺: [Ar]
To write an ion's configuration, write the atom's configuration, remove the 4s electrons first, then take any more from 3d.
Writing Fe³⁺ as [Ar]3d³4s². The 4s electrons go first, giving [Ar]3d⁵.
Section 4
Variable oxidation numbers
Transition metals show variable oxidation numbers, for example V (+2 to +5), Cr (+2, +3, +6), Mn (+2, +3, +4, +6, +7), Fe (+2, +3) and Cu (+1, +2).
The reason is that the 3d and 4s subshells are very close in energy. Both 4s and 3d electrons can be lost or used in bonding. Their successive ionisation energies rise gradually, and the energy released in forming ionic or covalent bonds compensates for the energy needed to remove each extra electron, so several oxidation numbers are stable.
Calcium is different: after losing its two 4s electrons, the third would come from the inner 3p subshell, which needs a very large energy, so calcium shows only +2. Zinc and scandium also show a single oxidation number, because their 3d subshells are full or empty in their ions.
Always link variable oxidation number to the similar energies of the 3d and 4s electrons.
Must Know
- Transition metal: a d-block element that forms one or more stable ions with an incomplete d-subshell.
- Sc (Sc³⁺, [Ar]) and Zn (Zn²⁺, 3d¹⁰) are not transition metals.
- 4s fills before 3d; Cr is [Ar]3d⁵4s¹ and Cu is [Ar]3d¹⁰4s¹.
- In ions the 4s electrons are lost first (Fe³⁺ is [Ar]3d⁵).
- Variable oxidation number: 3d and 4s energies are similar, so both can be used.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Properties of transition metals
- A student is classifying the period 4 d-block elements, from scandium (Z = 21) to zinc (Z = 30), using their atomic numbers.Explain why scandium is a d-block element but is not classified as a transition metal.2 marks
- Copper (Z = 29) is used in electrical wiring and forms compounds containing Cu⁺ ions and Cu²⁺ ions.Cu⁺ ions have a full 3d subshell. Explain why copper is nevertheless classified as a transition metal.2 marks
- Vanadium (Z = 23) forms compounds in which its oxidation number is +2, +3, +4 or +5, for example in VO₂⁺, which contains vanadium(V). Calcium (Z = 20) forms compounds in which its oxidation number is only +2.Deduce the full electronic configuration of a vanadium atom and of a V³⁺ ion, and state the number of 3d electrons in the vanadium in VO₂⁺.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).