The f-block elements are those in which the differentiating electrons enters the (n-2)f orbitals. There are two series of f-Block elements corresponding to filling of 4f and 5f-orbitals. The series of 4f-orbitals is called lanthanides. Lanthanides show different oxidation states depending upon stability of f⁰, f⁷ and f¹⁴ configurations, though the most common oxidation states is +3. There is a regular decrease in the size of lanthanides ions with increase in atomic number which is known as lanthanides contraction.

The atomic number of three lanthanides elements X, Y and Z are 65, 68 and 70 respectively. What is their Ln³⁺ electronic configuration?

 4f⁸, 4f¹¹, 4f¹³

₆₅Tb : [Xe]4f⁹6s²

Tb³⁺ :[Xe]4f⁸

₆₈Er : [Xe]4f¹²6s²

Er³⁺ : [Xe]4f¹¹

₇₀Yb : [Xe]4f¹⁴6s²

Yb³⁺ : [Xe]4f¹³