🗺️ Quick Snapshot

  • Lanthanoids are the 14 elements that follow lanthanum (Z = 57 – 71) and share many similar properties.:contentReference[oaicite:0]{index=0}
  • Each atom ends with the outer-shell pattern \(6s^{2}4f^{\,n}\) (where \(n = 1 \text{to} 14\)). Their most stable ions are tripositive, all having \(4f^{\,n}\) electronic cores.:contentReference[oaicite:1]{index=1}
  • A steady shrink in size across the series—called lanthanoid contraction—affects many later-period trends.

🎛️ Electronic Configuration

Every lanthanoid carries the same two electrons in the \(6s\) subshell, but the \(4f\) subshell fills step-by-step:

  • Neutral atom pattern: \([Xe]\,6s^{2}4f^{\,n}5d^{0\text{ or }1}\)
  • Tripositive ion pattern: \([Xe]\,4f^{\,n}\)  📌 most common oxidation state.:contentReference[oaicite:2]{index=2}

📏 Size Trend – Lanthanoid Contraction

Atomic and ionic radii drop smoothly from La to Lu because the 4f electrons shield the nuclear charge poorly. As a result, 3rd-row transition metals (e.g., Hf) end up almost the same size as their 2nd-row partners (e.g., Zr).

🔋 Oxidation States

  • \(+3\) dominates: Ln\(^{3+}\) compounds are the rule.:contentReference[oaicite:3]{index=3}
  • \(+4\) appears when an extra-stable configuration forms (e.g., Ce\(^{4+}\) with a noble-gas core). Ce\(^{4+}\) is such a strong oxidiser that water oxidation is possible, though very slow.:contentReference[oaicite:4]{index=4}
  • \(+2\) shows up in Eu\(^{2+}\), Sm\(^{2+}\) and Yb\(^{2+}\), thanks to half-filled or filled \(4f\) subshells. These ions act as strong reducing agents.:contentReference[oaicite:5]{index=5}

🧪 General Behaviour

  • Appearance: silvery-white, soft metals that tarnish quickly. Hardness grows across the series (samarium feels steel-hard).:contentReference[oaicite:6]{index=6}
  • Physical data: melting points cluster near 1000 – 1200 K (Sm: 1623 K); densities change smoothly except for Eu & Yb.:contentReference[oaicite:7]{index=7}
  • Many Ln\(^{3+}\) ions look brightly coloured due to f–f transitions; La\(^{3+}\) and Lu\(^{3+}\) stay colourless.:contentReference[oaicite:8]{index=8}
  • Magnetism: all Ln\(^{3+}\) ions except f0 (La\(^{3+}\), Ce\(^{4+}\)) and f14 (Yb\(^{2+}\), Lu\(^{3+}\)) are paramagnetic.:contentReference[oaicite:9]{index=9}
  • Ionisation enthalpies hover around 600 kJ mol-1 (1st) and 1200 kJ mol-1 (2nd); special stability at empty, half-filled and filled \(f\) subshells lowers the 3rd value for La, Gd, Lu.:contentReference[oaicite:10]{index=10}

⚗️ Common Chemical Reactions (see Fig. 4.7)

  • React gently with H2 to give hydrides.
  • Heat with carbon → carbides (Ln3C, Ln2C3, LnC2).
  • Displace H2 from dilute acids.
  • Burn in halogens → trihalides (LnX3).
  • Form basic oxides (Ln2O3) and hydroxides (Ln(OH)3).:contentReference[oaicite:11]{index=11}

Standard reduction pattern:
\(\mathrm{Ln}^{3+}_{(aq)} + 3e^- \;\longrightarrow\; \mathrm{Ln}_{(s)}\qquad E^\circ \approx -2.2\text{ to }-2.4 \text{ V (Eu: }-2.0\text{ V})\):contentReference[oaicite:12]{index=12}

🛠️ Everyday & Industrial Uses

  • Mischmetall (≈ 95 % lanthanoid metal + ≈ 5 % Fe) goes into lighter flints, Mg-alloy bullets & shells.:contentReference[oaicite:13]{index=13}
  • Mixed lanthanoid oxides catalyse petroleum cracking.:contentReference[oaicite:14]{index=14}
  • Certain individual oxides shine as phosphors in TV and LED screens.:contentReference[oaicite:15]{index=15}

🎯 High-Yield Points for NEET

  1. Lanthanoid contraction and its effect on later transition-metal sizes.
  2. Stability of the \(+3\) state vs. exceptional \(+2/+4\) states (Ce\(^{4+}\), Eu\(^{2+}\), etc.).
  3. Colour & magnetism linked to f–f transitions and unpaired \(4f\) electrons.
  4. Key reactions: hydride, carbide, oxide, halide formation; standard reduction potential pattern.
  5. Industrial relevance of mischmetall and lanthanoid catalysts/phosphors.

💡 Tip: remember the half-filled \(f^{7}\) and filled \(f^{14}\) “magic numbers” – they explain many odd oxidation states and magnetic behaviours!