Inner Transition Elements — Part 1: The lanthanoids and the lanthanoid contraction: visual lesson, ISC Class 12 Chemistry

Lesson

The f-block elements

Part 1 — the lanthanoids: fourteen elements that are almost chemical twins.

In this lesson you will

Write the general electronic configuration of the lanthanoids and of their tripositive ions
Explain the stability of the +3 state and the occurrence of +2 and +4 states
Describe the colour, complex formation and alloys of lanthanoids
Explain the cause of the lanthanoid contraction and its consequences

Schematic: the two bottom rows are the lanthanoids (4f) and the actinoids (5f).

Concept

Lanthanoids and actinoids: position

In the f-block, electrons enter an (n−2)f subshell. The lanthanoids are the fourteen elements cerium (58) to lutetium (71) that follow lanthanum and fill 4f; the actinoids, thorium (90) to lawrencium (103), fill 5f. Lanthanum and actinium are usually studied with them. Lanthanoids belong to group 3, period 6.

Table of the two f-block series
Series	Elements	Subshell filled	Period
Lanthanoids	Ce (58) to Lu (71)	4f	6
Actinoids	Th (90) to Lr (103)	5f	7

Concept

Electronic configurations of the lanthanoids

General configuration $[\text{Xe}]4f^{1\text{–}14}5d^{0\text{–}1}6s^2$ . The 4f and 5d levels are close in energy; a 5d electron appears only in La, Ce, Gd and Lu, where it keeps the 4f subshell empty, half-filled or full (Gd 4f⁷5d¹, Lu 4f¹⁴5d¹).

Table of the electronic configurations of selected lanthanoids
Element	Configuration	Element	Configuration
La (57)	[Xe]5d¹6s²	Gd (64)	[Xe]4f⁷5d¹6s²
Ce (58)	[Xe]4f¹5d¹6s²	Tb (65)	[Xe]4f⁹6s²
Pr (59)	[Xe]4f³6s²	Dy (66)	[Xe]4f¹⁰6s²
Nd (60)	[Xe]4f⁴6s²	Er (68)	[Xe]4f¹²6s²
Sm (62)	[Xe]4f⁶6s²	Yb (70)	[Xe]4f¹⁴6s²
Eu (63)	[Xe]4f⁷6s²	Lu (71)	[Xe]4f¹⁴5d¹6s²

Cookies