Chemistry · Periodic Classification Of Properties · NEET
| f-subshell filled | 4f (lanthanoids) | 5f (actinoids) |
| General configuration | [Xe] 4f^(1-14) 5d^(0-1) 6s^2 | [Rn] 5f^(1-14) 6d^(0-1) 7s^2 |
| Period | Period 6 (after La) | Period 7 (after Ac) |
| Common oxidation state | Mainly +3 | Variable: +3 to +6 (e.g. U up to +6) |
| Radioactivity | Mostly non-radioactive (only Pm) | All are radioactive |
| Contraction | Lanthanoid contraction (regular) | Actinoid contraction (larger, less regular) |
Lanthanoids fill the 4f subshell. Their general configuration is [Xe] 4f^(1-14) 5d^(0-1) 6s^2. Actinoids fill the 5f subshell. Their general configuration is [Rn] 5f^(1-14) 6d^(0-1) 7s^2. Both are called f-block or inner transition elements because the last electron enters an inner f-orbital, not the outer shell. This is why they are placed below the main table.
Actinoids have larger, less stable nuclei. All actinoids are radioactive because their nuclei have too many protons and neutrons to stay stable. Lanthanoids are mostly non-radioactive (only promethium, Pm, is radioactive). For NEET, remember: ALL actinoids are radioactive; almost ALL lanthanoids are stable. This is a common one-line MCQ.
In actinoids the 5f, 6d and 7s energy levels are very close in energy. So more electrons can take part in bonding, giving many oxidation states like +3, +4, +5, +6 (uranium shows up to +6). In lanthanoids the 4f electrons are held tightly and buried deep, so they mostly show only the stable +3 state. Rule to remember: lanthanoids = mainly +3; actinoids = variable, often higher.
As you move across the lanthanoids, the extra 4f electrons shield the nucleus poorly. So the effective nuclear charge slowly rises and the atomic and ionic size shrinks steadily. This is lanthanoid contraction. Its most tested effect: Zr and Hf (and Nb-Ta pairs) end up with almost the SAME size, so they behave very alike and are hard to separate. NEET has asked exactly this.
Transition (d-block) elements fill the (n-1)d subshell. Lanthanoids and actinoids fill an even deeper (n-2)f subshell — one shell further inside. Because the filling happens in an inner shell, they are called INNER transition elements. They sit inside period 6 (lanthanoids) and period 7 (actinoids).
Each series has 14 elements because the f-subshell holds a maximum of 14 electrons (7 f-orbitals x 2). Lanthanoids: Ce (58) to Lu (71), following lanthanum. Actinoids: Th (90) to Lr (103), following actinium. Some books count La and Ac inside the series, but the 14 f-filling elements are Ce-Lu and Th-Lr.
Zr (Z = 40) and Hf (Z = 72) have similar atomic and ionic radii because of:
The element Z = 114 has been discovered recently. It will belong to which family/group and electronic configuration?
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Yes. Every actinoid, from thorium (90) to lawrencium (103), is radioactive. In the lanthanoids only promethium (Pm) is radioactive; the rest are stable.
Lanthanoids fill the 4f subshell. Actinoids fill the 5f subshell. That is the single biggest difference between the two series.
Actinoids show more oxidation states (up to +6, e.g. uranium) because their 5f, 6d and 7s levels are close in energy. Lanthanoids mostly show only +3.
If placed inside periods 6 and 7 the table would become too wide. Since their last electron enters an inner f-orbital and their chemistry is very similar, all 14 are shown as a separate footnote row.
Fourteen. The f-subshell has 7 orbitals, holding a maximum of 14 electrons, so each series (Ce-Lu and Th-Lr) has 14 elements.
