Position in the periodic table and electronic configuration
d-block elements fill the (n−1)d orbitals and sit in groups 3 to 12. A transition element has an incompletely filled d subshell in the atom or a common ion.
Where the d- and f-blocks sit
The d-block is the large middle part of the periodic table, groups 3 to 12, between the s-block and the p-block. In these elements the electrons fill the d orbitals of the penultimate (second-last) shell, (n−1)d. There are four rows, called the 3d, 4d, 5d and 6d series. They are called transition elements because their properties lie between those of the reactive s-block metals and the p-block elements. The f-block has two series, the lanthanoids (4f, cerium to lutetium) and the actinoids (5f, thorium to lawrencium). They are placed in a separate panel at the bottom of the table and are called inner transition elements.
Electronic configurations of the 3d series
| Element | Z | Configuration |
|---|---|---|
| Scandium, Sc | 21 | [Ar] 3d¹ 4s² |
| Titanium, Ti | 22 | [Ar] 3d² 4s² |
| Vanadium, V | 23 | [Ar] 3d³ 4s² |
| Chromium, Cr | 24 | [Ar] 3d⁵ 4s¹ |
| Manganese, Mn | 25 | [Ar] 3d⁵ 4s² |
| Iron, Fe | 26 | [Ar] 3d⁶ 4s² |
| Cobalt, Co | 27 | [Ar] 3d⁷ 4s² |
| Nickel, Ni | 28 | [Ar] 3d⁸ 4s² |
| Copper, Cu | 29 | [Ar] 3d¹⁰ 4s¹ |
| Zinc, Zn | 30 | [Ar] 3d¹⁰ 4s² |
Why chromium and copper are different
The 3d and 4s orbitals have very similar energies. A half-filled (d⁵) or completely filled (d¹⁰) set of d orbitals is extra stable, because of higher exchange energy and a symmetrical arrangement of electrons. So in chromium one 4s electron moves into 3d to give 3d⁵ 4s¹ instead of 3d⁴ 4s². In copper, it gives 3d¹⁰ 4s¹ instead of 3d⁹ 4s².
Transition element or not?
Writing configurations of ions
- When a transition metal forms a positive ion, it loses its 4s electrons first, then 3d electrons.
- Fe [Ar] 3d⁶ 4s² gives Fe²⁺ [Ar] 3d⁶ and Fe³⁺ [Ar] 3d⁵.
- Mn [Ar] 3d⁵ 4s² gives Mn²⁺ [Ar] 3d⁵. Cu gives Cu⁺ [Ar] 3d¹⁰ and Cu²⁺ [Ar] 3d⁹.
- Sc³⁺ and Ti⁴⁺ are 3d⁰; Zn²⁺ is 3d¹⁰.
Iron, zinc and copper, found in pulses, nuts and whole grains, are d-block elements our bodies need in small amounts. Iron carries oxygen in haemoglobin, and zinc and copper help many enzymes work.
2 marks: “Why are Zn, Cd and Hg not regarded as transition elements? Why is copper regarded as one?” Zn, Cd and Hg have full d¹⁰ orbitals in the ground state and in their common +2 ions. Copper is 3d¹⁰ 4s¹, but Cu²⁺ is 3d⁹, a partly filled d subshell.
2 marks: “Write the electronic configurations of Cr (Z = 24) and Cu (Z = 29). Why are they not as expected?” Cr: [Ar] 3d⁵ 4s¹; Cu: [Ar] 3d¹⁰ 4s¹. 3d and 4s have similar energies, and half-filled and fully filled d subshells are extra stable.
1 marks: “Write the electronic configuration of Mn²⁺ (Z = 25).” [Ar] 3d⁵ (the two 4s electrons are lost).
Cr and Cu cheat: one 4s electron jumps to make d⁵ and d¹⁰. Ions lose 4s first.