The f-block elements are those in which the differentiating electron 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 state is +3. There is a regular decrease in the size of lanthanide ions with increase in atomic number which is known as lanthanide contraction.
Which of the following is not the electronic configuration of a lanthanide?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → [Xe]4f145d106s2
In the lanthanides (Z = 57 to 71) the differentiating electron enters the 4f orbitals, outside the [Xe] core (54 electrons). Their outer configurations run from 5d16s2 (La) to 4f145d16s2 (Lu), with at most one 5d electron. Adding up the electrons of each option gives its atomic number:
Option (1) [Xe]4f106s2 — The electronic configuration of a lanthanide.
Electron count: 54 + 10 + 2 = 66.
Element: dysprosium.
Option (2) [Xe]4f15d16s2 — The electronic configuration of a lanthanide.
Electron count: 54 + 1 + 1 + 2 = 58.
Element: cerium.
Option (3) [Xe]4f145d106s2 — Not the electronic configuration of a lanthanide.
Electron count: 54 + 14 + 10 + 2 = 80.
Element: mercury. The 4f set is full and the 5d set holds ten electrons, so this is the last member of the 5d transition series (Hf to Hg), which begins only after the lanthanides end at Lu.
Option (4) [Xe]4f75d16s2 — The electronic configuration of a lanthanide.
Electron count: 54 + 7 + 1 + 2 = 64.
Element: gadolinium, where the half-filled 4f7 set takes the next electron into 5d.
NCERT (2026-27 edition):
Table 4.9: Electronic Configurations and Radii of Lanthanum and Lanthanoids
The d- and f-Block Elements – “There are mainly four series of the transition metals, 3d series (Sc to Zn), 4d series (Y to Cd), 5d series (La and Hf to Hg) and 6d series which has Ac and elements from Rf to Cn. The two series of the inner transition metals; 4f (Ce to Lu) and 5f (Th to Lr) are known as lanthanoids and actinoids respectively.”