Lesson 1 of 6

Identity and electron configurations

Identify transition elements and form their ions.

A-Level 9476 (2026-2027)

I can apply the transition-element definition.Syllabus
Syllabus 9476, 13(a). Apply the transition-element definition.
I can write first-row atom and ion configurations.Syllabus
Syllabus 9476, 13(b). Write first-row atom and ion configurations.

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 understanding

Cobalt has proton number 27. What is the configuration of Co³⁺?

[Ar]3d7, after losing the 4s electrons and no more
[Ar]3d4 4s2, after losing three of the 3d electrons
[Ar]3d6, after losing both 4s electrons and then one 3d

Show the answer

[Ar]3d6, after losing both 4s electrons and then one 3d

Co is [Ar]3d7 4s2. Ions form by losing 4s electrons first, then 3d. 27 - 3 = 24 electrons, [Ar]3d6.