Chemistry · D And F Block Elements · NEET
| Orbital filled | 4f orbitals (lanthanoids) | 5f orbitals (actinoids) |
| Elements | Ce (58) to Lu (71) | Th (90) to Lr (103) |
| Common oxidation state | Mostly +3 only | +3 to +7 (wide range) |
| Why range differs | 4f buried, very different energy from 5d/6s | 5f, 6d, 7s have comparable energies |
| Radioactivity | Not radioactive (only Pm is) | All are radioactive |
| Contraction | Lanthanoid contraction (smaller step) | Actinoid contraction (greater step) |
| Ionisation enthalpy (early members) | Higher (4f held tightly) | Lower (5f diffuse, held loosely) |
Lanthanoids fill the 4f orbitals. They are the 14 elements from Ce (58) to Lu (71), coming after Lanthanum (La). Actinoids fill the 5f orbitals. They are the 14 elements from Th (90) to Lr (103), coming after Actinium (Ac). Simple trick: Lanthanoid = 4f (the smaller number), Actinoid = 5f (the bigger number). Both are called f-block because the last electron enters an f orbital.
This is the most asked NEET point. In lanthanoids, the 4f orbitals are buried deep inside, so their energy is very different from the 5d and 6s orbitals. So only the outer electrons are easily lost, giving mostly +3. In actinoids, the 5f, 6d and 7s orbitals have almost the SAME (comparable) energy. So electrons from all three can be removed to different levels. This is why actinoids show a wide range of oxidation states like +3, +4, +5, +6 and +7. NEET 2017 asked exactly this.
No. This is a common trap. All actinoids are radioactive because they have unstable nuclei (uranium, plutonium, thorium, etc.). Lanthanoids are NOT radioactive (except promethium, Pm, which is the only radioactive lanthanoid). So if a question says 'all these elements are radioactive', it points to actinoids, not lanthanoids.
Both mean a steady decrease in size across the series. But actinoid contraction is SLIGHTLY greater from element to element than lanthanoid contraction. Reason: the 5f electrons in actinoids shield (block) the nuclear pull very poorly, so the nucleus pulls the electrons in more strongly. NEET 2021 tested this exact statement. Lanthanoid contraction is famous for making Zr and Hf almost the same size.
Early actinoids have LOWER ionisation enthalpies than early lanthanoids. Reason: the 5f electrons are more diffuse (spread out) and are held less tightly than the 4f electrons in lanthanoids. So they are removed more easily. NEET 2023 asked this as an Assertion-Reason question and both statements were correct.
Yes, both lanthanoid and actinoid ions are often coloured and paramagnetic because of unpaired f-electrons (f-f transitions). So colour is NOT a good point to tell them apart. The reliable differences for NEET are: orbitals (4f vs 5f), oxidation states (few vs many), radioactivity (only actinoids all radioactive), and the amount of contraction.
The reason for greater range of oxidation states in actinoids is attributed to:
The incorrect statement among the following is:
Assertion (A): Ionisation enthalpies of early actinoids are lower than for early lanthanoids. Reason (R): Electrons are entering 5f orbitals in actinoids which experience greater shielding from nuclear charge.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Yes, the meaning is the same. IUPAC prefers the names 'lanthanoid' and 'actinoid' because the ending '-ide' usually means a negative ion. NCERT uses lanthanoid and actinoid, so use those spellings in your NEET exam.
Both have exactly 14 elements. Lanthanoids: Ce (58) to Lu (71). Actinoids: Th (90) to Lr (103). This is because each series fills the 14 spaces of the f subshell.
Promethium (Pm, Z = 61) is the only radioactive lanthanoid. In contrast, ALL actinoids are radioactive. This is a favourite NEET trap point.
For both, losing 3 electrons (the two ns and one (n-1)d or f electron) gives a stable ion. Lanthanoids stick almost only to +3. Actinoids also commonly show +3 but ALSO show +4, +5, +6, +7 because their 5f, 6d, 7s energies are close.
Revise 'What Are d-Block and f-Block Elements' next. It explains where lanthanoids and actinoids sit in the periodic table and why they are called the f-block, which makes this comparison much clearer.
