Revision: D and F Block Elements >> F-block Elements Chemistry HSC Science (General) 12th Standard Board Exam Maharashtra State Board

In the lanthanoids, the electrons are filling the 4f-subshell. On moving from left to right, the nuclear charge increases, and this increase is expected to be compensated by the increase in the magnitude of the shielding effect by the 4f-electrons. However, the f-electrons have very poor shielding effects. With an increasing atomic number in the lanthanoid series, there is a progressive decrease in the atomic as well as ionic radii of trivalent ions from La³⁺ to Lu^3+, and this is known as lanthanoid contraction.

Position and Introduction:

The f-block consists of two series:

1. Lanthanoids — fourteen elements from Ce (58) to Lu (71), following Lanthanum (La, 57)
2. Actinoids — fourteen elements from Th (90) to Lr (103), following Actinium (Ac, 89)

In lanthanides, electrons enter the penultimate (4f) and pre-penultimate subshells.

General configuration of lanthanoids: [Xe] 4f¹⁻¹⁴ 5d⁰⁻¹ 6s²

Complete Lanthanoid Table:

La (5d¹6s²), Gd (4f⁷5d¹6s²), and Lu (4f¹⁴5d¹6s²) have a 5d¹ electron — they fill 5d before filling 4f again, due to the stability of half-filled (4f⁷) configuration.

Physical State:

• All are silvery white metals with tensile strength; good conductors of heat and electricity.
• Density ranges from 6.77 to 9.74 g/cm³ and increases with atomic number.
• They readily form alloys with other metals, especially iron.

Oxidation States

• Most common and stable OS = +3
• Some exist in +2 (Sm²⁺, Eu²⁺, Tm²⁺, Yb²⁺) — because they achieve stability trying to reach +3 OS.
• Some exist in +4 (Ce⁴⁺, Pr⁴⁺, Tb⁴⁺, Dy⁴⁺) — because they try to approach +3 from +4; hence, these are good oxidising agents.
• Elements in +2 OS act as reducing agents; in +4 OS act as oxidising agents.

Chemical Behaviour:

Lanthanoids (Ln) react with:

• Lanthanoids react with boiling water to give a mixture of oxide and hydride.
• They combine with most non-metals at moderate temperatures.
• Alkalis have no action on them.

Actinoids are the 14 elements from Th (90) to Lr (103). General configuration: [Rn] 5f¹⁻¹⁴ 6d⁰⁻² 7s²

Complete Actinoid Table: