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8873 / 2027
Chemical Bonding

Lesson 3 of 3

Structure and evidence

Connect bonds to reactivity and measured properties.

A-Level 8873, revised syllabus (2026-2027)

  • I can explain bond energy and length.Syllabus

    Syllabus 8873, 2(k). Explain bond energy and length.

  • I can compare covalent bond reactivity.Syllabus

    Syllabus 8873, 2(l). Compare covalent bond reactivity.

  • I can describe crystalline lattice types.Syllabus

    Syllabus 8873, 2(m). Describe crystalline lattice types.

  • I can predict physical properties from structure.Syllabus

    Syllabus 8873, 2(n). Predict physical properties from structure.

  • I can infer structure from properties.Syllabus

    Syllabus 8873, 2(o). Infer structure from properties.

Compare the bond you are changing

Bond length, strength and polarity explain different aspects of reactivity.

Bond length is the equilibrium distance between the nuclei of two bonded atoms. Bond energy is the energy required to break one mole of a specified covalent bond in gaseous species; tabulated average bond energies average over molecular environments. Bond breaking is endothermic. For similar bonds, greater length usually means poorer overlap and a weaker bond.

Compare C-Cl, C-Br and C-I: the halogen atom becomes larger down the group, so the bond becomes longer and generally weaker. Breaking C-I is easier than breaking C-Cl, despite C-Cl being more polar. Polarity instead identifies electron-poor and electron-rich sites, which influences how a reagent approaches and interacts with a molecule.

Watch out for this

A student says: "C=C is a stronger bond than C-C, so alkenes are less reactive than alkanes." What is wrong with this?

Show the answer

Bond energy alone does not rank the reactivity of every molecule. A C=C double bond is stronger overall than a C-C single bond, yet its accessible pi electron density enables addition reactions. State which bond or electron-rich region is involved and which reaction is being compared.

Check your understanding

Why does the H-I bond have a lower bond energy than the H-Cl bond?

  1. I is larger, so the H-I bond is longer and the shared pair is held less strongly.
  2. I is more electronegative than Cl, so it pulls the shared pair away from H.
  3. HI has stronger forces between molecules, which weaken the H-I bond inside.
Show the answer

I is larger, so the H-I bond is longer and the shared pair is held less strongly.

The shared pair is further from the iodine nucleus and the orbital overlap is less effective. The attraction is weaker, so less energy is needed to break one mole of H-I bonds in the gas.

Check your understanding

1-iodobutane reacts with aqueous hydroxide ions faster than 1-chlorobutane does. Which is the best explanation?

  1. I is more electronegative than Cl, so it leaves more readily
  2. C-I is longer with a lower bond energy, so it breaks more easily
  3. C-I is more polar than C-Cl, so OH- attacks it more strongly
  4. C-Cl is weaker than C-I, so the chloro compound reacts last
Show the answer

C-I is longer with a lower bond energy, so it breaks more easily

The C-X bond must break in the reaction. C-I is the weakest carbon-halogen bond, and bond strength outweighs polarity here.

Turn structures into properties, then reverse the argument

Name the particles, attraction and charge carriers before drawing a conclusion.

Five crystalline patterns
SolidParticles and structureProperties explained
NaCl; MgOGiant ionic lattice; opposite ions attract. MgO contains 2+ and 2- ions.High melting points; fixed ions do not conduct in the solid; mobile ions conduct when molten. MgO has stronger attractions than NaCl.
IodineDiscrete I2 molecules in a molecular lattice.Comparatively low melting point: overcome intermolecular forces, not I-I bonds. No mobile charge carriers.
DiamondEach carbon bonds to four others in a three-dimensional covalent network.Hard, very high melting temperature; no mobile charged particles.
GraphiteEach carbon bonds to three others in a sheet; electrons are delocalised. Weaker attractions act between sheets.High melting temperature; sheets slide, giving softness; delocalised electrons conduct.
IceDiscrete water molecules joined into an open hydrogen-bonded lattice.Less dense than liquid water; melting disrupts hydrogen bonding, not O-H bonds.
CopperPositive ions in a lattice with delocalised electrons.Conducts when solid and molten; layers can slide while metallic attraction remains.

For melting, identify the attraction that must be overcome. For conductivity, identify mobile charged particles. For solubility, consider whether new solute-solvent attractions can compensate for attractions disrupted in both substances; "ionic" is not a guarantee of water solubility. For malleability, explain whether a displaced layer remains held together or brings like charges into unfavourable proximity.

Worked exampleUse two measurements together

Solid X has a high melting point. It is an electrical insulator as a solid, but its melt conducts. Suggest a structure.

Try it first. Then show the worked stepsHide the worked steps
  1. A high melting point indicates strong attractions, but is consistent with several structures.
  2. The molten sample has mobile charged particles.
  3. An ionic lattice supplies fixed ions in the solid and mobile ions in the melt.
  4. A metal would usually conduct as a solid; a simple molecular solid would usually melt much lower.
Answer

A giant ionic structure best fits both observations. Do not claim a specific compound from this evidence alone.

Worked exampleDistinguish two carbon structures

Two carbon solids both withstand high temperatures. Only one conducts electricity and leaves a slippery mark. Identify them.

Try it first. Then show the worked stepsHide the worked steps
  1. Conductivity implies mobile delocalised electrons: graphite.
  2. The slippery mark is consistent with graphite sheets sliding.
  3. The other can be diamond: a rigid 3-D network with all four outer electrons involved in localised bonds.
Answer

Graphite is the conducting, slippery solid; diamond is the non-conducting network. High melting temperature alone does not distinguish them.

Check your understanding

Iodine melts at a low temperature, although the I-I bond is a strong covalent bond. Why?

  1. The I-I bond is weaker than most covalent bonds, so melting breaks the molecules into atoms.
  2. Melting separates I2 molecules by overcoming weak forces between them; the I-I bonds stay intact.
  3. Iodine is a giant covalent lattice in which few of its bonds need to break to melt it.
Show the answer

Melting separates I2 molecules by overcoming weak forces between them; the I-I bonds stay intact.

Iodine is simple molecular. Only the weak instantaneous dipole attractions between molecules are overcome, so little energy is needed.