s- and p-Block Elements: MDCAT Notes and MCQs
Periodic properties
| Property | Across a period → | Down a group ↓ |
|---|---|---|
| Atomic radius | Decreases (↑ nuclear charge) | Increases (new shells) |
| Ionization energy | Increases (exceptions: Be > B, N > O; Mg > Al, P > S) | Decreases |
| Electron affinity | Generally increases | Generally decreases (Cl > F due to F's small size) |
| Electronegativity | Increases (F = 4.0 highest) | Decreases |
| Metallic character | Decreases | Increases |
- Covalent radius: half the distance between nuclei of two identical bonded atoms. Ionic radius: cation smaller than parent atom; anion larger. Bond length: distance between nuclei of two bonded atoms (C–C 1.54 Å, C=C 1.34 Å, C≡C 1.20 Å).
- Ionization energy: minimum energy to remove the most loosely held electron from a gaseous atom. 2nd IE > 1st IE. Noble gases have the highest IE in each period; He the highest overall.
- Electron affinity: energy released when an electron is added to a gaseous atom.
Blocks
s-block: groups 1–2 (ns1–2). p-block: groups 13–18 (ns2np1–6). d-block: groups 3–12, transition elements ((n−1)d1–10). f-block: lanthanides & actinides ((n−2)f).
Group I (alkali metals)
- Water: 2M + 2H2O → 2MOH + H2; reactivity increases down the group (Li slow, Na vigorous, K catches fire).
- Oxygen: Li → Li2O (normal oxide); Na → Na2O2 (peroxide); K, Rb, Cs → KO2 (superoxides).
- Chlorine: 2M + Cl2 → 2MCl (ionic, white solids).
- Stored under kerosene. Li shows diagonal relationship with Mg.
Group II (alkaline earth metals)
- Water: Be does not react; Mg reacts with hot water/steam (MgO + H2); Ca, Sr, Ba react with cold water → M(OH)2 + H2.
- Oxygen: form MO (Ba also forms BaO2); Mg burns with a dazzling white flame. BeO is amphoteric; others basic.
- Chlorine: M + Cl2 → MCl2; BeCl2 is covalent (small, highly polarizing Be2+).
Group IV (C, Si, Ge, Sn, Pb)
- Non-metal (C, Si) → metalloid (Ge) → metals (Sn, Pb). Valence ns2np2; oxidation states +4 and +2. Inert pair effect: +2 becomes more stable down the group (Pb2+ more stable than Pb4+; PbO2 is an oxidizing agent).
- Oxides: CO2, SiO2 acidic; GeO2, SnO2, PbO2 amphoteric. CO is neutral.
- Chlorides MCl4: CCl4 does not hydrolyse (no d-orbitals in C); SiCl4 hydrolyses readily → SiO2/H4SiO4 + HCl.
- Carbon shows maximum catenation; silicon forms silicates and silicones.
PMDC learning outcomes for this unit
- Periodic trends & group chemistry: 11.1 Define and explain the terms atomic radii, ionic radii, covalent radii, ionization energy, electron affinity, electronegativity, bond energy and bond length.
- Periodic trends & group chemistry: 11.2 Recognize the demarcation of the periodic table into s-block, p-block, d-block and f-block.
- Periodic trends & group chemistry: 11.3 Describe reactions of Group I elements with water, oxygen and chlorine.
- Periodic trends & group chemistry: 11.4 Describe reactions of Group II elements with water, oxygen and chlorine.
- Periodic trends & group chemistry: 11.5 Describe reactions of Group IV elements.