What are lanthanoids and actinoids?
They make up the f-block: here the last electron enters an f-orbital two shells inside the outer shell, the (n−2)f.
- Lanthanoids: the 14 elements after lanthanum, cerium (Ce, 58) to lutetium (Lu, 71). La is often studied with them.
- Actinoids: the 14 elements after actinium, thorium (Th, 90) to lawrencium (Lr, 103). Ac is studied with them.
They are placed in two rows below the main table so the table does not become very wide. They are also called inner transition elements.
Electronic configuration of lanthanoids and actinoids
General form: (n−2)f¹⁻¹⁴ (n−1)d⁰⁻¹ ns².
- Lanthanoids: [Xe] 4f¹⁻¹⁴ 5d⁰⁻¹ 6s². Example: Nd = [Xe] 4f⁴ 6s²; Gd = [Xe] 4f⁷ 5d¹ 6s² (half-filled 4f⁷ is stable); Lu = [Xe] 4f¹⁴ 5d¹ 6s².
- Actinoids: [Rn] 5f¹⁻¹⁴ 6d⁰⁻¹ 7s². Example: U = [Rn] 5f³ 6d¹ 7s²; Th = [Rn] 6d² 7s².
The outer shell (6s² or 7s²) is the same for the whole row. So the elements in a row are very similar and hard to separate.
In actinoids, 5f and 6d have very close energies, so electrons can sit in either. That makes their configurations less regular.
Oxidation states
Lanthanoids: +3 is the main state for all. It comes from losing 6s² and one 4f/5d electron.
- Some also show +2 or +4 when that gives an empty, half-filled or full 4f set: Ce⁴⁺ (4f⁰), Tb⁴⁺ (4f⁷), Eu²⁺ (4f⁷), Yb²⁺ (4f¹⁴).
- Ce⁴⁺ is a good oxidising agent (it wants to go back to +3). Eu²⁺ and Yb²⁺ are reducing agents.
Actinoids: many more states, because 5f, 6d and 7s electrons all have similar energies. U shows +3 to +6; Np and Pu reach +7. +3 is common, but early actinoids prefer higher states (Th +4, Pa +5, U +6).
Lanthanoid contraction: cause
From La to Lu, atomic and M³⁺ ionic radii get steadily smaller. This is the lanthanoid contraction.
Why? Each step adds one proton to the nucleus and one electron to the 4f shell. f-orbitals have spread-out, odd shapes, so they are very poor at shielding. The outer electrons feel a slightly bigger pull every time. Fourteen small pulls add up to a big shrink.
Story step 4 shows the M³⁺ balls shrinking from 103 pm to 86 pm.
Consequences of lanthanoid contraction
- 4d and 5d are the same size: Zr (160 pm) ≈ Hf (159 pm), Nb ≈ Ta, Mo ≈ W. So these pairs have very similar properties and occur together in nature and are hard to separate.
- Lanthanoids are hard to separate: sizes differ only a little, so their chemistry is nearly the same.
- Basic strength falls from La(OH)₃ to Lu(OH)₃: smaller ions hold OH⁻ more tightly (more covalent), so they give it up less.
- 5d metals become denser and have higher ionisation enthalpy than expected.
A similar but larger shrinking in the actinoids is called actinoid contraction, because 5f shields even worse than 4f.
General properties of lanthanoids
- Silvery-white soft metals that tarnish quickly in air.
- Reactive, much like calcium: they burn to give Ln₂O₃, react with water to give H₂ and Ln(OH)₃, react with acids to give H₂.
- Many Ln³⁺ ions are coloured (from f–f jumps) in solid and in solution; La³⁺, Lu³⁺ (4f⁰, 4f¹⁴) are colourless.
- Most are paramagnetic except f⁰ and f¹⁴ ions.
Comparison of lanthanoids and actinoids
| Feature | Lanthanoids | Actinoids |
|---|---|---|
| Orbital being filled | 4f | 5f |
| Oxidation states | Mainly +3 (a few +2, +4) | Many: +3 to +7 |
| Radioactivity | Only Pm is radioactive | All are radioactive |
| Complex formation | Weak tendency | Strong tendency |
| Oxo ions | Do not form | Form, e.g. UO₂²⁺, PuO₂²⁺ |
| Shielding and contraction | 4f shields poorly: lanthanoid contraction | 5f shields worse: bigger actinoid contraction |
| Basic nature of hydroxides | Less basic | More basic |
| Occurrence | Found in nature | Most are made in reactors (after U) |
Similarities: both mainly +3, both show contraction, both are reactive, electropositive metals, and both give coloured ions.
Uses of d- and f-block elements
- Iron and steel: buildings, bridges, tools. Stainless steel adds Cr and Ni.
- Copper: electric wires; alloys like brass and bronze. Coins use Cu–Ni alloys.
- Titanium: light, strong aircraft parts and body implants. TiO₂: white paint and sunscreen.
- Catalysts: V₂O₅ (sulphuric acid), Fe (ammonia), Ni (hydrogenation), TiCl₄ + Al(CH₃)₃ (Ziegler catalyst for polythene), PdCl₂ (Wacker process).
- Silver bromide (AgBr): photography, because it is light sensitive.
- Zinc, nickel, cadmium: batteries and cells (dry cell, Ni–Cd).
- Misch metal (about 95% lanthanoids + 5% Fe with some S, C, Ca, Al): lighter flints, bullets and shells; Mg alloyed with 3% misch metal makes jet engine parts.
- Lanthanoid oxides: polishing glass; mixed oxides as catalysts in petroleum cracking; phosphors in TV and LED screens.
- Actinoids: U and Pu as nuclear fuel; Th in older gas mantles and nuclear fuel.
Try it: see the contraction yourself
1. In free play, stay on the lanthanoid row. Before sliding, predict: will Er be bigger or smaller than Nd? Slide and read the radius.
2. Switch to the actinoid row and count the cubes for U and Pu. Compare with Nd and Sm.
3. Make a quick graph on paper: x = element number from La to Lu, y = M³⁺ radius from the readout. You will get a steadily falling line.
4. At home: strike a gas-lighter flint and see the sparks. Those sparks are burning bits of misch metal.
Key formulas and definitions
- Lanthanoids: [Xe] 4f¹⁻¹⁴ 5d⁰⁻¹ 6s²
- Actinoids: [Rn] 5f¹⁻¹⁴ 6d⁰⁻¹ 7s²
- Main oxidation state: +3 (Ln); +3 to +7 (An)
- Radius: La³⁺ ≈ 103 pm → Lu³⁺ ≈ 86 pm
- Zr ≈ 160 pm, Hf ≈ 159 pm (effect of lanthanoid contraction)
Worked examples
1. Write the electronic configuration of Gd (Z = 64) and Gd³⁺.
Step 1: Xe has 54 electrons, so 10 are left. Step 2: a half-filled 4f⁷ is stable, so Gd = [Xe] 4f⁷ 5d¹ 6s². Step 3: remove 6s² and 5d¹ for +3 → Gd³⁺ = [Xe] 4f⁷.
2. Why does Ce show +4 and Eu show +2?
Ce (4f¹ 5d¹ 6s²): losing 4 electrons gives Ce⁴⁺ = 4f⁰, an empty, stable f-set. Eu (4f⁷ 6s²): losing only 6s² gives Eu²⁺ = 4f⁷, half-filled and stable. Stable f⁰, f⁷ and f¹⁴ decide these extra states.
3. How many unpaired electrons are in Ce³⁺ (Z = 58)? Find its spin-only magnetic moment.
Ce = [Xe] 4f¹ 5d¹ 6s². Ce³⁺: remove 6s² and 5d¹ → 4f¹. So n = 1. Spin-only μ = √(1 × 3) = 1.73 BM.
4. Arrange La(OH)₃, Gd(OH)₃ and Lu(OH)₃ by decreasing basic strength.
Size of M³⁺ falls La > Gd > Lu (lanthanoid contraction). Smaller ions hold OH⁻ more tightly, making the M–OH bond more covalent and less basic. So La(OH)₃ > Gd(OH)₃ > Lu(OH)₃.
5. Why are Zr and Hf so hard to separate?
Hf comes after the 14 lanthanoids. The lanthanoid contraction cancels the size increase expected from an extra shell, so Hf (159 pm) is almost the same size as Zr (160 pm). Same size and same outer electrons → nearly identical chemistry.
6. Which is a stronger oxidising agent in water: Ce⁴⁺ or Ce³⁺? Explain.
Ce⁴⁺. The normal, most stable state of lanthanoids is +3. Ce⁴⁺ takes an electron to go back to Ce³⁺, so it oxidises other substances.
Common mistakes
- Saying the lanthanoid contraction is caused by adding shells. No new shell is added; it is due to poor shielding by 4f electrons as nuclear charge rises.
- Thinking all lanthanoids show many oxidation states like actinoids. Lanthanoids mainly show +3; only a few give +2 or +4.
- Forgetting that 4d and 5d radii are almost equal (Zr ≈ Hf). Many students expect 5d to be much bigger.
- Saying all lanthanoids are radioactive. Only promethium (Pm) is; all actinoids are radioactive.