S2.4 From models to materialsIB Chemistry SL: Revision notes

Real bonds are rarely purely ionic, covalent or metallic. Bonding is best described as a continuum between the three models, shown on a bonding triangle (in the data booklet).

• Horizontal axis: average electronegativity of the two elements.
• Vertical axis: electronegativity difference (ΔEN).
• Ionic vertex: large ΔEN (e.g. CsF). Metallic vertex: low average, small ΔEN (e.g. Cs, Mg). Covalent vertex: high average, small ΔEN (e.g. F₂, Cl₂).

A compound's position shows the relative contributions of each bonding type.

Worked example: AlCl₃ (Al 1.6, Cl 3.2) has average (1.6 + 3.2) ÷ 2 = 2.4 and ΔEN = 1.6, so it lies in the polar covalent region, not the ionic region — consistent with it forming molecules and subliming at a low temperature.

Predictions from position:

• Ionic region: high melting point, brittle, conducts when molten or in solution.
• Covalent region: does not conduct; molecular substances have low melting points, network solids high.
• Metallic region: conducts as a solid, malleable, lustrous.

An alloy is a mixture of a metal with other metals or non-metals, e.g. steel (Fe + C), brass (Cu + Zn), bronze (Cu + Sn), duralumin (Al + Cu + Mg). Alloys have enhanced properties — typically harder, stronger or more corrosion resistant.

Because metallic bonding is non-directional, atoms of different sizes can be incorporated into the lattice and remain bonded to the delocalised electrons. The different-sized atoms disrupt the regular layers, so layers cannot slide easily, making the alloy harder. The delocalised electrons remain, so alloys still conduct.

A composite combines two or more distinct materials, each keeping its own structure, to give properties neither has alone. In reinforced concrete, concrete (ionic and covalent network solids) is strong in compression but brittle in tension, while steel (metallic) is strong in tension. Together they make beams that resist both. Other examples: fibreglass (glass fibres in a polymer) and carbon-fibre-reinforced polymers.

Polymers are large molecules (macromolecules) made of many repeating subunits called monomers. Plastics share common properties explained by their structure:

• Long chains of strong, non-polar C–C and C–H covalent bonds → unreactive and non-biodegradable.
• No ions or delocalised electrons → electrical insulators.
• Chains held together by relatively weak London forces → low density, soften on heating, flexible.

Unbranched chains (HDPE) pack closely → stronger London forces → denser and higher softening temperature than branched chains (LDPE).

Addition polymers form when one bond of the C=C double bond in each monomer breaks and the monomers join into a chain of single bonds. Nothing else is produced.

To draw the repeating unit: take the two carbons of the double bond, make it a single bond, keep all substituents, and add bonds extending out of brackets.

• ethene CH₂=CH₂ → –[CH₂–CH₂]–
• propene CH₂=CHCH₃ → –[CH₂–CH(CH₃)]–
• chloroethene CH₂=CHCl → –[CH₂–CHCl]–
• tetrafluoroethene CF₂=CF₂ → –[CF₂–CF₂]–

Number of monomers in a chain = molar mass of chain ÷ molar mass of monomer.