From Models To Materials

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Bonding Models

Bonding models simplify complex systems and allow prediction of substance properties.
Chemical bonding broadly falls into three types: ionic, covalent and metallic.
Real bonding situations are more complex than these three idealised types suggest.
Example: aluminium chloride, AlCl3, contains a metal and a non-metal, so it would be expected to be ionic with a high melting point.
However, AlCl3 melts at 192 °C, which is lower than expected for an ionic compound, due to covalent character.
The bonding in AlCl3 is polar covalent, not purely ionic.

The Bonding Triangle

Bonding is better visualised using a bonding triangle, which places compounds on a spectrum.
The triangle uses electronegativity difference (∆χ) on the y-axis and average electronegativity (χ̄) on the x-axis.
Difference in electronegativity: ∆χ = χA − χB.
Average electronegativity: χ̄ = (χA + χB) / 2.
Elements have zero Δχ and appear along the x-axis depending on their electronegativity.
Metals have zero Δχ and appear along the x-axis, but near the metallic corner.
Ionic compounds have large Δχ and are placed near the apex of the triangle.
Covalent compounds with low Δχ are near the bottom right; polar covalent compounds lie in between ionic and covalent.

Using the Bonding Triangle

The bonding triangle appears in Section 17 of the IB Chemistry Data Booklet; electronegativity values are in Section 9.
You don’t need to memorise the bonding triangle; focus on interpreting it when needed.
Each compound is plotted using values in the format (x, y).
Sodium (Na) has an electronegativity of 0.9; as a pure element Δχ = 0, placing it in the bottom left, 100% metallic.
Chlorine (Cl2) has an electronegativity of 3.2; as a diatomic molecule Δχ = 0, placing it in the bottom right, 100% covalent.
Sodium chloride (NaCl) has average electronegativity Σχ = (3.2 + 0.9)/2 = 2.05 and Δχ = 3.2 − 0.9 = 2.3, placing it near the apex with around 75% ionic character.
This explains NaCl’s high melting point and ability to conduct electricity when molten.

Percentage of Bonding Type

The triangular bonding diagram can help estimate the percentage of ionic or covalent character in a compound.
Aluminium chloride (AlCl3): Σχ = (1.6 + 3.2)/2 = 2.4, Δχ = 3.2 − 1.6 = 1.6, placing it at (2.4, 1.6) with 50% ionic character.
Aluminium oxide (Al2O3): Σχ = (1.6 + 3.4)/2 = 2.5, Δχ = 3.4 − 1.6 = 1.8, placing it at (2.5, 1.8) with 60% ionic character.
Both AlCl3 and Al2O3 fall near the polar covalent region, indicating significant ionic character and covalent properties.
Al2O3 has a much higher melting point (2072 °C) due to stronger ionic bonding; AlCl3 melts at just 192 °C due to weaker covalent interactions.
The diagram allows accurate assessment of real bonding behaviour and prediction of properties like melting point, solubility and electrical conductivity.
You do not need to calculate exact percentage ionic character in exams; use the triangle to compare materials qualitatively.

Properties of Alloys

An alloy is a mixture of metals, or a metal mixed with a non-metal such as carbon.
The elements are physically combined but not chemically bonded; atoms of different elements are spread throughout the metallic lattice.
Atoms are held together by delocalised electrons, as in pure metals; metallic bonds are non-directional, allowing atoms of different sizes to occupy the lattice.
In a pure metal, metal cations are arranged in regular layers, allowing them to slide past one another easily, making the metal malleable.
In alloys, atoms of different sizes disrupt this regular pattern; the distorted lattice makes it harder for the layers to slide.
This gives alloys increased hardness and strength compared to pure metals.
Alloys may also be more corrosion resistant or resistant to extreme temperatures.
Examples: brass (copper and zinc), steel (iron with carbon), stainless steel (iron, chromium, nickel, carbon), solder (lead and tin), bronze (copper and tin).

Structure of an alloy

Polymers

Polymers are large molecules made by chemically linking many small molecules called monomers.
A polymer must contain at least 50 repeating units; each unit is joined to the next by strong covalent bonds.
Polymers are also known as macromolecules due to their large size compared with simple molecules.
Some polymers contain just one type of monomer unit, e.g. poly(ethene) and poly(chloroethene) (PVC).
Others contain two or more different types of monomer units and are called copolymers, e.g. nylon and biological proteins.
Natural polymers include proteins, starch and DNA; DNA forms a double helix with millions of linked nucleotides.
Synthetic polymers include plastics like poly(ethene) and nylon; they are man-made and widely used due to durability, low reactivity and water resistance.
Synthetic polymers are generally non-biodegradable because of their chemical stability, leading to long-lasting pollution.

Monomers joining to form a polymer

Addition Polymers

Addition polymerisation involves monomers containing at least one C=C double bond joining together to form a long-chain polymer.
The only product is the polymer; no by-products are formed.
The π-bond in each C=C bond breaks and allows monomers to link by forming new C–C single bonds.
Examples: poly(ethene) from ethene monomers; poly(chloroethene) or PVC from chloroethene monomers.
A repeat unit is the smallest group of atoms that repeats to form the polymer chain; it is shown inside square brackets [ ] with an 'n' outside.
In poly(alkenes), the repeat unit is the same as the monomer except the C=C becomes a C–C bond.
The structure inside square brackets is the repeat unit – not the monomer; the monomer is the same as the repeat unit but with C=C bonds instead of C–C bonds.
Properties of polymers: low density (chains loosely packed), unreactive (chemically stable chains), water-resistant (repel water), strong (held by strong covalent bonds).

Addition polymerisation

Condensation Polymers (HL)

In condensation polymerisation, monomers join together and release a small molecule as a by-product, most commonly water or hydrogen chloride.
Condensation polymers often contain either ester links (–COO–) or amide links (–CONH–).
Ester links are formed between a carboxylic acid and an alcohol group; commonly found in polyesters.
Amide links are formed between a carboxylic acid and an amine group; commonly found in polyamides such as proteins and nylon.
A polyester is formed from diols and dicarboxylic acids, or from hydroxycarboxylic acids; e.g. PET (Terylene or Dacron).
A polyamide is formed from diamines and dicarboxylic acids, diamines and dioyl dichlorides, or amino acids; e.g. nylon-6,6.
The amide link (–CONH–) is called a peptide bond when formed between amino acids.
Polyesters and polyamides can be broken by hydrolysis because ester/amide bonds are susceptible to water attack; this is a major advantage over addition polymers.

Comparing Addition and Condensation Polymerisation

Addition polymerisation uses monomers with carbon–carbon double bonds and forms no by-products.
Addition polymers like poly(ethene) have saturated carbon backbones and cannot be hydrolysed, so they persist in the environment.
Condensation polymerisation uses monomers with two different functional groups (e.g. –COOH and –NH2) and releases a small molecule like water as a by-product.
Condensation polymers like nylon and proteins contain amide or ester links between the monomers.
Condensation polymers are strong, biodegradable (some can break down over time) and versatile – can be tailored for flexibility, toughness or water resistance.
Hydrolysis adds water and causes the polymer to break down into the original monomers; both condensation and hydrolysis reactions are controlled by enzymes.

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Practice questions

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1.Which type of bonding is found at the apex (top) of a triangular bonding diagram?
Easy
- AIonic bonding
- BCovalent bonding
- CMetallic bonding
- DPolar covalent bonding
2.What is represented on the x-axis of a standard triangular bonding diagram?
Easy
- ADifference in electronegativity (Δχ)
- BAverage electronegativity (χ̄)
- CIonic character (%)
- DMelting point
3.An alloy is a mixture of a metal with another element. Which statement about alloys is correct?
Easy
- AAlloys contain atoms of all the same size
- BAlloys are softer than the pure metal they originate from
- CAlloys are harder than the pure metal they originate from
- DAlloys have a regular lattice that allows layers to slide easily
4.In addition polymerisation, a small molecule such as water is released as a by-product.
Easy
True or false?
5.A polymer must contain at least 50 repeating units.
Easy
True or false?
6.Which of the following statements about alloys are correct? (select all that apply)
Medium
- AAlloys contain atoms of all the same sizes
- BAn alloy is a mixture of a metal with another element
- CAlloys are harder than the pure metal they originate from
- DAlloys have a regular lattice that allows layers to slide easily
- EAlloys are held together by delocalized electrons
7.Which pair of monomers is used to form a polyester?
Medium
- AA diamine and a dicarboxylic acid
- BA diol and a dicarboxylic acid
- CTwo amino acids
- DA dioyl dichloride and a diamine
8.Which linkage is formed in a polyamide?
Medium
- AEster link (–COO–)
- BAmide link (–CONH–)
- CPeptide link (–COO–)
- DEther link (–O–)

More questions

9.Match each polymer with the type of polymerisation used to form it.
Medium
- Poly(ethene)
- Nylon-6,6
- PET (Terylene)
- Addition polymerisation
- Condensation polymerisation (polyamide)
- Condensation polymerisation (polyester)
10.Arrange the following steps in the correct order to show how a polyamide is formed by condensation polymerisation.
Medium
- A hydrogen from the diamine –NH2 group reacts with an –OH group from the dicarboxylic acid
- A water molecule is eliminated as a by-product
- An amide link (–CONH–) forms between the monomers
- The monomers join together to form a long-chain polyamide
11.Match each polymer with the linkage or feature that correctly describes it.
Medium
- Polyester
- Polyamide
- Poly(ethene)
- Protein
- Ester link (–COO–)
- Amide link (–CONH–)
- Saturated carbon backbone, no by-product
- Natural polyamide of amino acids
12.Which statements about condensation polymerisation are correct? (select all that apply)
Medium
- AA small molecule such as water or hydrogen chloride is released as a by-product
- BMonomers must contain at least one C=C double bond
- CAn amide link is formed between a carboxylic acid group and an amine group
- DThe only product formed is the polymer
- EPolyesters can be broken down by hydrolysis
13.Arrange the steps to show how a polyester is formed by condensation polymerisation, starting from the monomers.
Medium
- A hydrogen atom is removed from the –OH group of the diol
- The –OH group is removed from the –COOH group of the dicarboxylic acid
- A water molecule is eliminated as a by-product
- An ester link (–COO–) forms between the monomer units
14.Which statement best describes the position of elements on a triangular bonding diagram?
Medium
- AElements have zero Δχ and lie along the x-axis according to their electronegativity
- BElements have large Δχ and lie near the apex of the triangle
- CElements have zero average electronegativity and lie along the y-axis
- DElements are plotted using the electronegativity difference of their compound
15.Which of the following are natural polymers? (select all that apply)
Medium
- ADNA
- BStarch
- CNylon
- DPoly(ethene)
- EProteins
16.Which of the following is the monomer used to form poly(chloroethene), PVC?
Easy
- AChloroethene
- BChloroethane
- CEthene
- D2-chloropropene
17.Which statement correctly compares addition and condensation polymerisation?
Medium
- AAddition polymerisation forms no by-product, while condensation polymerisation releases a small molecule
- BAddition polymerisation releases water, while condensation polymerisation releases no by-product
- CBoth types of polymerisation release water as a by-product
- DAddition polymerisation uses monomers with two different functional groups, while condensation uses C=C bonds
18.Brass is an alloy of copper (χ = 1.9) and zinc (χ = 1.6). Which pair of values (average electronegativity, electronegativity difference) correctly places brass on the bonding triangle?
Medium
- A(1.75, 0.3)
- B(0.3, 1.75)
- C(3.5, 0.3)
- D(1.75, 3.5)
19.Aluminium chloride, AlCl3, melts at 192 °C, far lower than expected for an ionic compound. Which statement best explains this?
Medium
- AThe bonding has significant covalent character, so the bonding is polar covalent rather than purely ionic
- BThe compound is metallic and its delocalised electrons weaken the lattice
- CThe aluminium–chlorine bonds are non-polar, giving a simple covalent structure
- DAluminium chloride has a giant covalent structure with weak intermolecular forces
20.Which statement about the bonding triangle is correct?
Easy
- AIonic compounds are placed near the apex because they have a large electronegativity difference
- BCovalent compounds are placed near the apex because they have a large electronegativity difference
- CMetallic elements are placed at the apex because they have zero electronegativity difference
- DPolar covalent compounds are placed at the bottom left because they have the lowest average electronegativity
21.An element X has an electronegativity of 2.5. Where is X plotted on a triangular bonding diagram?
Medium
- A(0, 2.5)
- B(2.5, 0)
- C(2.5, 2.5)
- D(0, 0)
22.A substance is plotted at (2.1, 0.2) on a triangular bonding diagram. Which description best fits this substance?
Medium
- AA metallic element with some covalent character
- BAn ionic compound with about 75% ionic character
- CA purely covalent compound with no metallic character
- DAn alloy with a very large electronegativity difference
23.Use the electronegativity values: Mg = 1.2, Cl = 3.2, O = 3.4. Which compound has the larger percentage ionic character?
Medium
- AMgO, because it has the larger electronegativity difference (2.2)
- BMgCl2, because it has the larger electronegativity difference (2.0)
- CMgO, because it has the smaller average electronegativity (2.3)
- DMgCl2, because it contains two chlorine atoms per magnesium
24.The electronegativity of boron is 2.0 and of bromine is 2.8. Using the bonding triangle, which bonding type is predicted for BBr3?
Hard
- APolar covalent
- BPurely ionic
- CPurely metallic
- DNon-polar covalent
25.The diagram shows part of a polymer chain with the repeat unit –CH2–CHCl–. Which monomer formed this polymer?
Medium
- AChloroethene
- BChloroethane
- C1,2-dichloroethane
- DEthene
26.A section of an addition polymer has the repeat unit –CH2–CH(OH)–. Which monomer was used to make this polymer?
Medium
- AEthenol, CH2=CHOH
- BEthane-1,2-diol, HOCH2CH2OH
- CEthanal, CH3CHO
- DProp-2-en-1-ol, CH2=CHCH2OH
27.A polyester is formed from ethane-1,2-diol and benzene-1,4-dicarboxylic acid. Which small molecule is released as a by-product?
Medium
- AWater
- BHydrogen chloride
- CCarbon dioxide
- DHydrogen
28.A polyamide is made from hexane-1,6-diamine and a dioyl dichloride rather than a dicarboxylic acid. Which by-product is formed?
Hard
- AHydrogen chloride
- BWater
- CCarbon dioxide
- DAmmonia
29.Which property is characteristic of nylon but NOT of poly(ethene)?
Medium
- AIt can be broken down by hydrolysis
- BIt is held together by strong covalent bonds
- CIt is formed from monomers containing C=C double bonds
- DIt has a saturated carbon backbone
30.A student compares nylon-6,6 and nylon-6,12, which are both polyamides made from hexane-1,6-diamine. Which statement best explains why nylon-6,12 has a lower melting point?
Hard
- AIt has longer hydrocarbon chains between the amide links, reducing hydrogen bonding between chains per unit mass
- BIt has more amide links per unit mass, which increases hydrogen bonding
- CIt contains ester links instead of amide links
- DIt is formed by addition polymerisation rather than condensation polymerisation
31.Which of the following is a natural condensation polymer?
Medium
- AProtein
- BPoly(ethene)
- CNylon-6,6
- DPoly(chloroethene)
32.Alloys contain atoms of different sizes, which disrupts the regular lattice and makes the alloy harder than the pure metal.
Easy
True or false?
33.Poly(ethene) can be broken down by hydrolysis in landfill sites.
Easy
True or false?
34.In an addition polymerisation, the only product is the polymer.
Easy
True or false?
35.Which of the following statements about the triangular bonding diagram are correct? (select all that apply)
Medium
- AElements are found along the x-axis because their electronegativity difference is zero
- BIonic compounds are found near the apex of the triangle
- CMetallic substances are found at the bottom right of the triangle
- DCovalent compounds with low electronegativity difference are found near the bottom right
- EThe x-axis represents the difference in electronegativity
36.Which of the following statements about condensation polymers are correct? (select all that apply)
Hard
- APolyesters contain ester links (–COO–)
- BPolyamides contain amide links (–CONH–)
- CCondensation polymerisation releases a small molecule such as water
- DCondensation polymers cannot be hydrolysed
- ECondensation polymers are formed from monomers containing C=C double bonds
37.Which of the following are properties of addition polymers such as poly(ethene)? (select all that apply)
Medium
- ALow density
- BWater-resistant
- CUnreactive
- DBiodegradable
- EContain ester links
38.Match each polymer or process with the correct linkage or feature.
Medium
- Polyester
- Polyamide
- Addition polymer
- Protein
- Ester link (–COO–)
- Amide link (–CONH–)
- Saturated carbon backbone with no by-products
- Peptide links between amino acids
39.Match each monomer type with the polymer it can form.
Medium
- Diol + dicarboxylic acid
- Diamine + dicarboxylic acid
- Alkene with C=C double bond
- Hydroxycarboxylic acid
- Polyester
- Polyamide
- Addition polymer
- Polyester such as PHB
40.Arrange the following in order of increasing percentage ionic character, from lowest to highest.
Medium
- A purely covalent element with Δχ = 0
- A polar covalent compound with Δχ = 0.8
- A compound with Δχ = 1.6
- An ionic compound with Δχ = 2.3
41.Name the small molecule released as a by-product when a polyamide is formed from a diamine and a dicarboxylic acid.
Easy
42.Calculate the average electronegativity (χ̄) of caesium iodide, CsI, given that caesium has an electronegativity of 1.0 and iodine has an electronegativity of 2.7.
Medium
43.Calculate the difference in electronegativity (Δχ) for aluminium oxide, Al2O3, given that aluminium has an electronegativity of 1.6 and oxygen has an electronegativity of 3.4.
Medium
44.Calculate the average electronegativity (Σχ) of caesium iodide, CsI, given that caesium has an electronegativity of 1.0 and iodine has an electronegativity of 2.7.
Medium
45.Complete the sentences about condensation polymers.
Medium
A polyester contains the ____ link, formed between a carboxylic acid and an alcohol group, while a polyamide contains the ____ link, formed between a carboxylic acid and an amine group.
46.Give the name of the linkage formed when a polyamide is produced.
Easy
47.State the type of polymerisation used to form poly(phenylethene).
Easy
48.Calculate the average electronegativity Σχ of boron trifluoride, BF3, given that boron has an electronegativity of 2.0 and fluorine has an electronegativity of 4.0.
Medium
49.Calculate the electronegativity difference Δχ for magnesium oxide, MgO, given that magnesium has an electronegativity of 1.2 and oxygen has an electronegativity of 3.4.
Medium
50.Poly(2-chloropropene) is formed by the addition polymerisation of 2-chloropropene (C3H5Cl, Mr = 76.5). Calculate the percentage atom economy for this polymerisation. Give your answer to the nearest whole number.
Medium
51.A polyamide is formed from 1.00 mol of hexane-1,6-diamine (Mr = 116) and 1.00 mol of a dicarboxylic acid (Mr = 146). The polymerisation releases water (Mr = 18). Calculate the mass, in g, of water released when 1.00 mol of each monomer reacts completely.
Medium
52.Complete the sentences about the triangular bonding diagram.
Easy
The x-axis of the triangular bonding diagram represents the ____ electronegativity, while the y-axis represents the ____ in electronegativity.
53.Explain why alloys are generally harder than the pure metal they are made from.
Hard
54.Explain why poly(ethene) is unreactive and water-resistant.
Hard
55.Explain why poly(phenylethene) is less reactive than Terylene.
Hard
56.Explain why a plasticiser such as bis(2-ethylhexyl) adipate increases the flexibility of PVC.
Hard
57.Calculate the average electronegativity (χ̄) of caesium iodide, CsI, given that caesium has an electronegativity of 1.0 and iodine has an electronegativity of 2.7.
Medium
- A0.85
- B1.85
- C2.85
- D3.85
58.Calculate the difference in electronegativity (Δχ) for aluminium oxide, Al2O3, given that aluminium has an electronegativity of 1.6 and oxygen has an electronegativity of 3.4.
Medium
- A3.8
- B2.8
- C0.8
- D1.8
59.Calculate the average electronegativity (Σχ) of caesium iodide, CsI, given that caesium has an electronegativity of 1.0 and iodine has an electronegativity of 2.7.
Medium
- A0.85
- B1.85
- C2.85
- D3.85
60.Calculate the average electronegativity Σχ of boron trifluoride, BF3, given that boron has an electronegativity of 2.0 and fluorine has an electronegativity of 4.0.
Medium
- A2
- B3
- C4
- D5
61.Calculate the electronegativity difference Δχ for magnesium oxide, MgO, given that magnesium has an electronegativity of 1.2 and oxygen has an electronegativity of 3.4.
Medium
- A1.2
- B3.2
- C2.2
- D4.2
62.Poly(2-chloropropene) is formed by the addition polymerisation of 2-chloropropene (C3H5Cl, Mr = 76.5). Calculate the percentage atom economy for this polymerisation. Give your answer to the nearest whole number.
Medium
- A90 %
- B100 %
- C120 %
- D110 %
63.A polyamide is formed from 1.00 mol of hexane-1,6-diamine (Mr = 116) and 1.00 mol of a dicarboxylic acid (Mr = 146). The polymerisation releases water (Mr = 18). Calculate the mass, in g, of water released when 1.00 mol of each monomer reacts completely.
Medium
- A40 g
- B32 g
- C44 g
- D36 g

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