Topic 1 of 6

Identity and electron configurations

Identify transition elements and form their ions.

A-Level 9476 (2026-2027)

Which elements count as transition elements?

Use the d subshell of the atom and its cations, not just its position on the table.

A transition element is a d-block element whose atom has an incomplete d subshell, or which forms a cation with an incomplete d subshell. Incomplete means between one and nine d electrons. The definition tests electron configuration; being coloured or forming a complex is supporting behaviour, not the definition.

Three useful boundary cases
Element	Configuration evidence	Conclusion
Sc	Sc is [Ar]3d¹4s²; Sc³⁺ is [Ar].	Included by its atom, although its usual +3 ion is d⁰.
Cu	Cu is [Ar]3d¹⁰4s¹; Cu²⁺ is [Ar]3d⁹.	Included by the incomplete d subshell in Cu²⁺.
Zn	Zn is [Ar]3d¹⁰4s²; its usual Zn²⁺ ion is [Ar]3d¹⁰.	A d-block element, but excluded from the transition elements in this course.

For first-row transition-metal cations, write the neutral atom first, then remove 4s electrons before 3d electrons. The order used to build a neutral-atom configuration is not a universal order of removal. Chromium and copper are the familiar neutral-atom exceptions to a simple 4s² pattern.

First-row configurations: the [Ar] core is omitted in every entry
Element	Atom after [Ar]	Example ion after [Ar]
Sc	3d¹4s²	Sc³⁺: 3d⁰
Ti	3d²4s²	Ti³⁺: 3d¹
V	3d³4s²	V³⁺: 3d²
Cr	3d⁵4s¹	Cr³⁺: 3d³
Mn	3d⁵4s²	Mn²⁺: 3d⁵
Fe	3d⁶4s²	Fe²⁺: 3d⁶; Fe³⁺: 3d⁵
Co	3d⁷4s²	Co²⁺: 3d⁷
Ni	3d⁸4s²	Ni²⁺: 3d⁸
Cu	3d¹⁰4s¹	Cu⁺: 3d¹⁰; Cu²⁺: 3d⁹

Check your understandingCobalt has proton number 27. Give the configuration of Co³⁺ and explain the removal order.Think it through, then reveal the answer

Co is [Ar]3d⁷4s². Remove both 4s electrons and then one 3d electron: Co³⁺ is [Ar]3d⁶. The electron count is 24, as required by 27 - 3; removing three 3d electrons first would give the wrong ion.