Q: What does H2 Chemistry Notes: CORE IDEA 2, Topic 2 - Chemical Bonding cover?
A: Consolidate VSEPR shapes, bond polarity, intermolecular forces, and lattice energy trends for Core Idea 2 (Chemical Bonding) in the 2026 H2 Chemistry syllabus.
Chemical bonding connects atomic structure to macroscopic behaviour: lattice energies govern melting points, molecular geometry decides intermolecular forces, and bond polarity predicts reaction pathways. This note organises the skills required for Paper 2 and Paper 3 questions, supported by exam-style worked examples. For the full set of H2 Chemistry revision resources, refer to https://eclatinstitute.sg/blog/h2-chemistry-notes.
1 Types of Chemical Bonds
Bonding model
Key features
Typical exam focus
Ionic
Electrostatic attraction between cations and anions in a lattice.
Explaining trends in melting point, solubility, conductivity.
Covalent
Shared pair(s) of electrons between atoms.
Bond polarity, molecular shape, sigma vs pi bonds.
Metallic
Lattice of positive ions in a sea of delocalised electrons.
Conductivity, malleability, strength across the periodic table.
1.1 Lattice Energy Trends
Lattice energy measures enthalpy change when one mole of ionic solid separates into gaseous ions. A simplified Born-Landé equation highlights core dependencies:
The lattice energy U is roughly proportional to (z+×z−
) divided by
r0
, where z₊ and z₋ are the ionic charges and r₀ is the internuclear distance. Larger charge products and shorter separations produce more exothermic lattice energies, raising melting points and hardness.
1.2 Covalent Bond Strength and Length
Stronger bonds have higher bond dissociation enthalpies and shorter lengths. The syllabus expects students to relate strength to orbital overlap: sigma bonds involve head-on overlap (stronger), whereas pi bonds involve side-on overlap (weaker). Recall that double bonds comprise one sigma and one pi bond.
2 Molecular Geometry and Polarity
2.1 VSEPR Workflow
Determine the central atom's steric number: count bonded pairs + lone pairs.
Assign electron-domain geometry (e.g. tetrahedral for steric number 4).
Deduce molecular shape by considering only bonding pairs.
Judge polarity: combine vector sum of bond dipoles; consider symmetry.
Steric number
Electron-domain geometry
Common shapes
Bond angles
3
Trigonal planar
Trigonal planar, bent
\(\pu{120^\circ}\), \(\pu{<120^\circ}\)
4
Tetrahedral
Tetrahedral, trigonal pyramidal, bent
\(\pu{109.5^\circ, 107^\circ, 104.5^\circ}\)
5
Trigonal bipyramidal
Seesaw, T-shaped, linear
\(\pu{120^\circ}\) and \(\pu{90^\circ}\) combinations
6
Octahedral
Square pyramidal, square planar
\(\pu{90^\circ}\)
2.2 Bond Polarity and Molecular Dipole
Even if bonds are polar, molecules can be non-polar when dipoles cancel (e.g. COX2). Use vector addition language: “the dipoles are equal and opposite along the linear axis.” For molecules such as NHX3, asymmetry yields a net dipole, linking to hydrogen bonding strength.
3 Intermolecular Forces (IMF)
IMF
Requirements
Relative strength
Example
London dispersion
Present in all molecules; induced dipoles.
Weakest; increases with molecular mass and surface area.
\(\ce{I2}\)
Permanent dipole-dipole
Polar molecules with permanent dipoles.
Medium.
\(\ce{CH3Cl}\)
Hydrogen bonding
\(\ce{H}\) bonded to \(\ce{N}\), \(\ce{O}\), or \(\ce{F}\); lone pair on adjacent molecule.
Strongest IMF; drives high boiling points.
\(\ce{H2O}\), \(\ce{HF}\)
Candidates should support IMF arguments with both electron distribution (polarity) and ability to form specific interactions (e.g. availability of lone pairs).
4 Worked Multi-Part Example
Question:
Consider PClX3 and PClX5.
Predict the shapes using VSEPR.
Explain why PClX5 exists whereas NClX5 does not.
Compare the melting points of PClX3 and PClX5
Solution outline:
PClX3: Steric number 4 (three bonding pairs + one lone pair) → trigonal pyramidal; bond angle 107∘.
PClX5: Steric number 5 → trigonal bipyramidal.
Phosphorus can expand its octet because vacant 3d orbitals are accessible for bonding, whereas nitrogen lacks low-energy d orbitals and has minimal spatial extent, preventing NCl5 formation.
PClX5 forms an ionic lattice in the solid state ([PClX4]X+[PClX6]X−
Each explanation links shape, orbital availability, and lattice structure-the exact reasoning markers want.
5 Exam Tactics
Paper 1: Expect geometry MCQs where lone pairs must be counted correctly. Sketch quick Lewis structures to avoid errors.
Paper 2: Structured questions often request energy comparisons (e.g. lattice energies of MgO vs NaF). Reference ionic charge and ionic radius explicitly, stating relative charge density.
Paper 3: Long questions integrate bonding with properties. For example, deducing why silicon dioxide is a solid network while carbon dioxide remains gaseous at RTP-discuss covalent network vs discrete molecules.
Paper 4: Practical planning tasks may need justification of solvents based on polarity or explanation of observed conductivity based on ionic vs molecular structure.
6 Frequent Misconceptions
Confusing bond polarity with molecular polarity: Always consider shape.
Forgetting lone-pair repulsion: Bond angles decrease with each lone pair; quote specific angle adjustments.
Misstating metallic bonding explanations: Highlight delocalised electrons and cation lattice rather than vague “sea of electrons” statements.
Incorrect octet expansion reasoning: Emphasise availability of low-energy d orbitals and size effect, not the outdated “d-orbital participation” myth; modern interpretation focuses on energetics and delocalised bonding.
7 Quick Drill Set
Compare the melting points of MgClX2, NaCl, and AlX2OX3 using lattice energy arguments.
Sketch and label the shapes of SFX4, IFX5
Explain why graphite conducts electricity in the plane but diamond does not, referencing bonding and structure.
Worked solutions should mention the bonding model, structural arrangement, and the inter-particle forces in each case.
Chemical bonding answers carry high-value reasoning marks because they bridge microscopic structure and macroscopic properties. Rehearse the language used here for fluency in Papers 2 and 3, and revisit the master hub at https://eclatinstitute.sg/blog/h2-chemistry-notes for supplementary drills.
