Bonding and materials

Earlier topics treated bonding as three neat boxes: ionic, covalent and metallic. In reality bonding is a continuum - most substances sit somewhere between the extremes.

The van Arkel-Ketelaar triangle (given in data booklet section 17) maps this continuum. Each substance is plotted using two numbers worked out from electronegativity values (data booklet section 9):

• x-axis = the average electronegativity $\bar\chi = \dfrac{\chi_A + \chi_B}{2}$
• y-axis = the electronegativity difference $\Delta\chi = \chi_A - \chi_B$ (always take the larger minus the smaller, so $\Delta\chi \ge 0$)

The three corners are:

• Metallic - bottom left (low $\bar\chi$, small $\Delta\chi$): e.g. Na, Cu.
• Covalent - bottom right (high $\bar\chi$, small $\Delta\chi$): e.g. $\text{Cl}_2$, $\text{SiO}_2$.
• Ionic - top (large $\Delta\chi$): e.g. NaCl, MgO.

A large $\Delta\chi$ pushes a substance up towards ionic; a small $\Delta\chi$ keeps it along the bottom edge, with $\bar\chi$ deciding metallic vs covalent. Substances near the edges and middle (e.g. some semiconductors and polar covalent compounds) have intermediate character.

Once you have plotted a substance, its region lets you predict structure and physical properties.

• Ionic region (top): giant ionic lattice. High melting point, brittle, conducts only when molten or aqueous (ions free to move), often soluble in water.
• Covalent region (bottom right): either small molecules (low melting point, non-conducting, e.g. $\text{CO}_2$) or covalent networks (very high melting point, hard, e.g. diamond, $\text{SiO}_2$). The triangle alone does not tell you which - you also need the formula and structure.
• Metallic region (bottom left): giant metallic lattice. Conducts as solid and liquid, malleable and ductile, lustrous.

Working method:

1. Look up each element's $\chi$ (section 9).

2. Calculate $\bar\chi$ and $\Delta\chi$.

3. Plot on the triangle and read off the region.

4. State the structure, then the expected properties.

Watch out: the triangle gives the dominant bonding type, not a yes/no answer. A compound such as $\text{AlCl}_3$ sits in an intermediate zone (significant covalent character despite a metal + non-metal), which is why it has a low sublimation point - exactly the kind of nuance the triangle is designed to reveal.

An alloy is a mixture of a metal with one or more other elements (usually other metals). It is not a compound - there is no fixed ratio and no new chemical bond type.

In a pure metal, identical positive ions sit in regular layers within a sea of delocalised electrons. Applying a force lets layers slip over each other, so pure metals are relatively soft.

In an alloy, atoms of different sizes are mixed in. These disrupt the regular layers, so the layers can no longer slide easily. The metallic bonding (delocalised electrons) is retained, but the alloy is:

• harder and stronger than the pure metal
• often more resistant to corrosion

Examples:

• Bronze = copper + tin (harder than copper)
• Brass = copper + zinc
• Steel = iron + carbon (small non-metal atoms wedge into the lattice)
• Solder = tin + lead (low melting point)

Because the electron sea is unchanged, alloys still conduct electricity and heat and remain malleable, just less easily deformed than the pure metal. A simple particle diagram shows two sizes of cation in a single delocalised electron sea.

A polymer is a very large molecule (a macromolecule) made by linking many small monomers into long chains. Plastics are synthetic polymers.

The repeating unit is the part that recurs along the chain; we write polymers with this unit in brackets and a subscript $n$ to show it repeats many times.

Properties depend on:

• Chain length - longer chains tangle more, giving stronger, higher-melting plastics.
• Branching - straight (linear) chains pack closely, so they are denser and stronger (e.g. HDPE); branched chains pack loosely, giving softer, lower-density plastics (e.g. LDPE).
• Intermolecular forces between chains - stronger forces (e.g. hydrogen bonding in nylon) raise the melting point and strength.

Many plastics are thermoplastics: the chains are held together only by intermolecular forces, so they soften on heating and can be remoulded and recycled.

Polymers can be natural (proteins, cellulose, DNA) or synthetic (poly(ethene), PVC, nylon). The big environmental issue is that many synthetic plastics are non-biodegradable because their strong C-C backbones resist breakdown.

Addition polymers are made from unsaturated monomers - molecules containing a $\text{C=C}$ double bond. The double bond opens up and the monomers join end to end with no atoms lost.

General pattern for an ethene-type monomer:

$$n\,\text{CH}_2\text{=CH}_2 \longrightarrow {-}(\text{CH}_2{-}\text{CH}_2){-}_n$$

Key features:

• The repeat unit contains exactly the same atoms as the monomer (the same molecular formula) - nothing is eliminated.
• The product backbone is a saturated C-C chain.

How to draw the repeat unit:

1. Draw the monomer with the $\text{C=C}$ horizontal.

2. Change the double bond to a single bond.

3. Extend a bond from each carbon out through the brackets.

4. Add the subscript $n$.

Common examples (monomer -> polymer):

• ethene -> poly(ethene)
• chloroethene -> PVC (poly(chloroethene))
• tetrafluoroethene -> PTFE (Teflon)
• propene -> poly(propene)

Given a polymer you can also work backwards to the monomer: find the repeat unit, then put a $\text{C=C}$ back between the two backbone carbons.

Condensation polymers form when each monomer carries two reactive functional groups, so chains can grow from both ends, and a small molecule is eliminated at every new link (usually $\text{H}_2\text{O}$).

Polyesters form from a diol + a dicarboxylic acid (or a single monomer with both an -OH and a -COOH). The link is an ester group:

$$\text{-COOH} + \text{HO-} \longrightarrow \text{-COO-} + \text{H}_2\text{O}$$

Example: ethane-1,2-diol + benzene-1,4-dicarboxylic acid -> PET (Terylene).

Polyamides form from a diamine + a dicarboxylic acid. The link is an amide (peptide) group, and water is again released:

$$\text{-COOH} + \text{H}_2\text{N-} \longrightarrow \text{-CONH-} + \text{H}_2\text{O}$$

Example: nylon-6,6 from a 6-carbon diamine + a 6-carbon diacid.

Contrast with addition polymers:

• Addition: monomer has C=C; no small molecule lost; repeat unit = monomer atoms.
• Condensation: monomers have two functional groups; a small molecule is lost each link; repeat unit is lighter than the combined monomers.

Proteins (amino acids joined by amide/peptide bonds) and polysaccharides are natural condensation polymers.