Topic 17 · page 1 of 20

Transition metals and their electrons

A transition metal is defined by its ions, not its atoms. Work out the electron configuration of an ion first, then ask whether its 3d sub-shell is partly filled.

12 min read17.117.2

Configurations of Sc to Zn 17.2

The ten elements from scandium to zinc fill the 3d sub-shell. They are the d-block elements of Period 4. Three rules give every configuration:

Element Atomic number Configuration
Sc 21 [Ar] 3d¹ 4s²
Ti 22 [Ar] 3d² 4s²
V 23 [Ar] 3d³ 4s²
Cr 24 [Ar] 3d⁵ 4s¹
Mn 25 [Ar] 3d⁵ 4s²
Fe 26 [Ar] 3d⁶ 4s²
Co 27 [Ar] 3d⁷ 4s²
Ni 28 [Ar] 3d⁸ 4s²
Cu 29 [Ar] 3d¹⁰ 4s¹
Zn 30 [Ar] 3d¹⁰ 4s²

Writing 4s before 3d is also correct. Choose one order and keep to it.

Configurations of ions 17.2

A positive ion has fewer electrons than its atom. The 4s electrons are in the outer shell, so they are lost first, and 3d electrons only after that.

MethodConfiguration of a d-block ion

  1. Number of electrons = atomic number − charge.
  2. Write the configuration of the atom.
  3. Remove electrons from 4s first, then from 3d, until you reach the number in step 1.
  4. Check that the superscripts add up to the number of electrons.

Worked exampleFe²⁺ and Fe³⁺

  1. Iron, Z = 26, is [Ar] 3d⁶ 4s².
  2. Fe²⁺ has 24 electrons: remove the two 4s electrons. Fe²⁺ is [Ar] 3d⁶.
  3. Fe³⁺ has 23 electrons: remove one more, from 3d. Fe³⁺ is [Ar] 3d⁵.

Worked exampleCr³⁺ and Cu²⁺

  1. Chromium, Z = 24, is [Ar] 3d⁵ 4s¹. Cr³⁺ has 21 electrons. Remove the 4s electron, then two from 3d. Cr³⁺ is [Ar] 3d³.
  2. Copper, Z = 29, is [Ar] 3d¹⁰ 4s¹. Cu²⁺ has 27 electrons. Remove the 4s electron, then one from 3d. Cu²⁺ is [Ar] 3d⁹.
  3. Copper(I) loses only the 4s electron: Cu⁺ is [Ar] 3d¹⁰.

What a transition metal is 17.1

Many textbooks say a transition metal is "any element in the d-block". That is not good enough for the exam. The definition is about the ions.

A sub-shell is incompletely filled if it holds some electrons but not the maximum. For 3d the maximum is ten, so an ion qualifies when it has between 1 and 9 electrons in 3d.

Ion Electrons in 3d Incompletely filled?
Sc³⁺ 0 No: empty
Ti³⁺ 1 Yes
Fe²⁺ 6 Yes
Cu²⁺ 9 Yes
Zn²⁺ 10 No: full
Five empty or filled 3d boxes for each of five ions. Sc3+ has no electrons in its 3d boxes, labelled empty. Ti3+ has one electron, Fe2+ has six and Cu2+ has nine, each labelled incompletely filled. Zn2+ has ten, all five boxes full, labelled full.
The 3d boxes of five ions. Only the ions with some, but not all, 3d places filled count towards the definition.

Scandium forms only Sc³⁺, which is 3d⁰. Zinc forms only Zn²⁺, which is 3d¹⁰. Neither forms an ion with a partly filled 3d sub-shell, so neither is a transition metal, although both are in the d-block.

Copper is a transition metal because Cu²⁺ is 3d⁹. It does not matter that Cu⁺ is 3d¹⁰. One stable ion with an incompletely filled 3d sub-shell is enough.

Check yourself

  1. Give the full electronic configuration of vanadium (Z = 23) and of V³⁺.
  2. Give the configurations of Cu (Z = 29) and Cu²⁺, using [Ar].
  3. Give the configuration of Mn²⁺ (Z = 25) and of Ni²⁺ (Z = 28), using [Ar].
  4. Write the configuration of Cr³⁺ (Z = 24). A student gives [Ar] 3d¹ 4s². Explain the mistake.
  5. Zinc is in the d-block. Explain why it is not a transition metal.
  6. Define a transition metal. Use the definition to explain why iron is one.

Answers cover these until you have tried the questions

  1. V: 1s² 2s² 2p⁶ 3s² 3p⁶ 3d³ 4s². V³⁺ has 23 − 3 = 20 electrons. The 4s electrons go first, then one 3d electron: 1s² 2s² 2p⁶ 3s² 3p⁶ 3d².
  2. Cu is [Ar] 3d¹⁰ 4s¹. Cu²⁺ has 27 electrons: lose 4s¹, then one 3d electron: [Ar] 3d⁹.
  3. Mn is [Ar] 3d⁵ 4s², so Mn²⁺ loses both 4s electrons: [Ar] 3d⁵. Ni is [Ar] 3d⁸ 4s², so Ni²⁺ is [Ar] 3d⁸.
  4. Cr is [Ar] 3d⁵ 4s¹. Cr³⁺ has 21 electrons: lose 4s¹, then two 3d: [Ar] 3d³. The student's answer has the right number of electrons (21) but arranged as in a neutral scandium atom. In a d-block ion the 4s sub-shell is emptied first, so no electrons stay in 4s.
  5. Zinc forms only the ion Zn²⁺, which is [Ar] 3d¹⁰. The 3d sub-shell is full, so zinc forms no stable ion with an incompletely filled d sub-shell.
  6. A transition metal is a d-block element that forms one or more stable ions with an incompletely filled d sub-shell. Iron forms Fe²⁺ ([Ar] 3d⁶) and Fe³⁺ ([Ar] 3d⁵), and both have a partly filled 3d sub-shell.