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 ideal types suggest.
Example: Aluminium chloride (AlCl₃) contains a metal and non-metal, so it would be expected to be ionic with a high melting point, but it melts at 192 °C due to covalent character; its bonding is polar covalent, not purely ionic.

Bonding as a Continuum

Bonding is better visualised using a bonding triangle, which places compounds on a spectrum.
The triangle uses electronegativity difference (Δχ) on the y-axis and average electronegativity (χ̄) on the x-axis.
Δχ = χ_A − χ_B; χ̄ = (χ_A + χ_B) / 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 near the metallic corner (bottom left).
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.
Bonding character is based on electronegativity trends, not just position in the Periodic Table.

Using the Triangular Bonding Diagram

The triangular bonding diagram is in Section 17 of the IB Chemistry Data Booklet; electronegativity values are in Section 9.
The diagram classifies bonds as a continuum between ionic (top), covalent (bottom right), and metallic (bottom left).
Each compound is plotted using (x, y) = (average electronegativity, electronegativity difference).
Example: Sodium (Na) has electronegativity 0.9 and Δχ = 0, placing it at the bottom left (100% metallic).
Example: Chlorine (Cl₂) has electronegativity 3.2 and Δχ = 0, placing it at the bottom right (100% covalent).
Example: Sodium chloride (NaCl) has χ̄ = (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 its high melting point and ability to conduct electricity when molten.

Percentage of Bonding Type

The triangular bonding diagram can estimate the percentage of ionic or covalent character in a compound.
Example: Aluminium chloride (AlCl₃) has χ̄ = (1.6 + 3.2)/2 = 2.4 and Δχ = 3.2 − 1.6 = 1.6, placing it at (2.4, 1.6) with 50% ionic character.
Example: Aluminium oxide (Al₂O₃) has χ̄ = (1.6 + 3.4)/2 = 2.5 and 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.

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 and are held together by delocalized electrons, as in pure metals.
Metallic bonds are non-directional, allowing atoms of different sizes to occupy the lattice.
In a pure metal, cations are arranged in regular layers that slide past one another easily, making the metal malleable.
In alloys, atoms of different sizes disrupt the regular pattern, making it harder for layers to slide, so alloys are harder and stronger than pure metals.
Alloys may also be more corrosion resistant or resistant to extreme temperatures.
Examples: Brass (copper and zinc) is strong and corrosion resistant; Steel (iron with carbon, often chromium, vanadium, molybdenum) is very strong; Stainless steel (iron, chromium, nickel, carbon) is corrosion resistant; Solder (lead and tin) has a low melting point; Bronze (copper and tin) is hard and corrosion resistant.

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 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 Polymerisation

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, allowing monomers to link by forming new C–C single bonds.
Examples: Poly(ethene) from ethene monomers; Poly(chloroethene) (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.
To deduce the monomer from a polymer, identify the repeat unit (2 carbons in the main chain) and replace the C–C bond with a C=C bond.

Addition polymerisation

Properties of Polymers

Low density – polymer chains are loosely packed, so plastics are lightweight compared to metals or ceramics.
Unreactive – most plastics don’t react easily because the polymer chains are chemically stable.
Water-resistant – plastics repel water and don’t absorb moisture, making them ideal for containers and packaging.
Strong – polymers are held together by strong covalent bonds, making many plastics tough and durable.
These properties make polymers useful in everyday materials such as packaging, clothing, construction products, and transportation components.

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Übungsfragen

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1.Which of the following statements about alloys are correct? I. Alloys contain atoms of all the same sizes II. An alloy is a mixture of a metal with another element III. Alloys are harder than the pure metal they originate from
Easy
- AI and II only
- BI and III only
- CII and III only
- DI, II and III
2.A molecule of a polymer contained the sequence shown. Which monomer could produce this polymer by addition polymerisation?
Medium
- ACHCl=CHCl
- BCH2=CHCl
- CCH3CCl=CHCl
- DCH3CCl=CH2
3.Which of the following chemicals is not correctly placed within this triangular bonding diagram?
Easy
- ALi
- BO2
- CCH4
- DCBr4
4.Which type of bonding can be described as 'the electrostatic attraction between positive nuclei and electrons and occurs by the sharing of electrons'?
Medium
- AHydrogen bonding
- BIonic bonding
- CMetallic bonding
- DCovalent bonding
5.A binary compound is plotted on a triangular bonding diagram. The electronegativity values for each element are 1.6 and 3.4. Which estimate for the percentage ionic and percentage covalent character in the compound is correct?
Medium
- A10% covalent, 90% ionic
- B40% covalent, 60% ionic
- C55% covalent, 45% ionic
- D100% covalent, 0% ionic
6.The properties of alloys can be explained in terms of metals having I. Non-directional bonding II. Delocalised electrons III. Ions of different size
Medium
- AI and II only
- BI and III only
- CII and III only
- DI, II and III
7.Using Sections 9 and 17 of the Data Booklet, which of the following predictions about gallium tribromide are incorrect? I. It is a poor insulator. II. It has a high melting and boiling point. III. It is hard and brittle.
Medium
- AI and II only
- BI and III only
- CII and III only
- DI, II and III
8.Match each region of the triangular bonding diagram with the type of bonding found there.
Easy
- Apex (top) of the triangle
- Bottom right of the triangle
- Bottom left of the triangle
- Ionic bonding
- Covalent bonding
- Metallic bonding

Mehr Fragen

9.Order these steps to show how a binary compound is plotted on a triangular bonding diagram.
Medium
- Look up the electronegativity values of both elements in Section 9 of the Data Booklet
- Calculate the average electronegativity, Σχ, of the two elements
- Calculate the electronegativity difference, Δχ, between the two elements
- Plot the point (Σχ, Δχ) on the bonding triangle in Section 17 of the Data Booklet
10.Which of the following statements about addition polymerisation are correct? (select all that apply)
Medium
- AThe monomers contain a C=C double bond
- BThe only product is the polymer
- CA small molecule such as water is eliminated as a by-product
- DThe π-bond in each C=C bond breaks and new C–C single bonds form
- EThe monomer and the repeat unit are always identical in every way
11.In a pure metal, the regular layers of cations can slide past one another easily, which makes the metal malleable.
Easy
True or false?
12.Which of the following best describes why an alloy is generally harder than the pure metal it is made from?
Medium
- AAtoms of different sizes disrupt the regular lattice, making it harder for layers to slide
- BThe alloy contains strong covalent bonds between the different metal atoms
- CThe alloy has a lower density because the atoms are more loosely packed
- DThe delocalised electrons are removed when a second element is added
13.Which of the following are properties of alloys compared with pure metals? (select all that apply)
Medium
- AAlloys are generally harder
- BAlloys are generally more malleable
- CAlloys may be more corrosion resistant
- DAlloys are held together by ionic bonds
- EAlloys may be more resistant to extreme temperatures
14.Which statement about the bonding in aluminium chloride is correct?
Medium
- AIt is purely ionic because it contains a metal and a non-metal
- BIt is polar covalent, which gives it a lower melting point than expected for an ionic compound
- CIt is purely metallic because aluminium is a metal
- DIt is a simple covalent molecule with no ionic character at all
15.For caesium iodide, caesium has an electronegativity of 1.0 and iodine has an electronegativity of 2.7. Which point (Σχ, Δχ) represents caesium iodide on the triangular bonding diagram?
Hard
- A(1.85, 1.7)
- B(1.7, 1.85)
- C(3.7, 1.7)
- D(1.85, 3.7)
16.Which of the following statements about the triangular bonding diagram are correct? (select all that apply)
Hard
- AElements have zero Δχ and appear along the x-axis
- BThe x-axis shows the average electronegativity of the elements
- CThe y-axis shows the difference in electronegativity between the two elements
- DIonic compounds have small Δχ and are placed near the bottom of the triangle
- ECovalent compounds with low Δχ are found near the bottom right of the triangle
17.Natural polymers include proteins, starch and DNA.
Easy
True or false?
18.Which of the following is the best explanation for why poly(ethene) is water-resistant?
Medium
- AThe polymer chains are chemically stable and do not absorb moisture
- BThe polymer contains many ionic bonds that repel water
- CThe polymer chains are held together by hydrogen bonds to water
- DThe polymer has a low density, so water cannot enter the structure
19.A polymer has the repeat unit shown: the main chain has two carbons, one carbon bears a chlorine atom and the other bears a chlorine atom, with no other substituents. Which monomer formed this polymer by addition polymerisation?
Medium
- ACHCl=CHCl
- BCH2=CCl2
- CCCl2=CCl2
- DCH2=CHCl
20.A polymer chain has the repeat unit –CH2–CH(Cl)–. Which statements about this polymer are correct? (select all that apply)
Medium
- AIt is formed from chloroethene monomers
- BIt is poly(chloroethene), also known as PVC
- CIt is formed by condensation polymerisation with water as a by-product
- DIts monomer contains a C=C double bond
- EThe repeat unit contains a C=C double bond
21.Place the following materials in order of increasing percentage ionic character, from most covalent to most ionic.
Medium
- Chlorine, Cl2
- Brass (copper–zinc alloy)
- Aluminium chloride, AlCl3
- Sodium chloride, NaCl
22.Which quantity is plotted on the y-axis of a triangular bonding diagram?
Medium
- AAverage electronegativity
- BElectronegativity difference
- CNumber of covalent bonds
- DMelting point
23.A substance is plotted at an average electronegativity of 2.4 and an electronegativity difference of 1.6. In which region of the triangular bonding diagram does it lie?
Medium
- AMetallic
- BPolar covalent
- C100% covalent
- D100% ionic
24.Substance X is a binary compound with an electronegativity difference of 1.7 and an average electronegativity of 1.85. Which type of bonding does X most likely have?
Hard
- AMetallic
- BPurely covalent
- CIonic
- DHydrogen bonding
25.Silicon tetrachloride, SiCl4, is a simple molecular substance. Which statements about SiCl4 compared with an ionic substance of similar formula mass are correct? (select all that apply)
Hard
- AIt has a lower boiling point
- BIt does not conduct electricity when molten
- CIt has a giant ionic lattice
- DIt is held together by weak intermolecular forces between molecules
- EIt has a high melting point due to strong ionic bonding
26.Substance A is an alloy containing a Group 1 metal and a Group 2 metal. Using electronegativity data, which pair of metals could form this alloy?
Hard
- ACaesium (1.0) and magnesium (1.3)
- BSodium (0.9) and chlorine (3.2)
- CCopper (1.9) and zinc (1.6)
- DIron (1.8) and carbon (2.5)
27.A student plots a binary compound on a triangular bonding diagram. Which axes are used?
Medium
- Ax-axis: average electronegativity; y-axis: difference in electronegativity
- Bx-axis: difference in electronegativity; y-axis: average electronegativity
- Cx-axis: atomic number; y-axis: electronegativity difference
- Dx-axis: average electronegativity; y-axis: melting point
28.A binary compound has an electronegativity difference of 1.6 and an average electronegativity of 2.4. Using the bonding triangle, which region does it fall into?
Medium
- APolar covalent
- BPure covalent
- CMetallic
- DIonic
29.A student is asked to plot caesium iodide on a triangular bonding diagram. Caesium has an electronegativity of 1.0 and iodine has an electronegativity of 2.7. Which of the following statements are correct? (select all that apply)
Hard
- AThe average electronegativity is 1.85.
- BThe electronegativity difference is 1.7.
- CThe point (1.85, 1.7) lies in the ionic region.
- DThe compound is expected to have a low melting point.
- ECaesium iodide is a covalent compound.
30.In an alloy, the atoms of different sizes disrupt the regular lattice, making it harder for layers to slide.
Easy
True or false?
31.Place the following steps in the correct order to plot a binary compound on a triangular bonding diagram.
Medium
- Find the electronegativity values of the two elements from Section 9.
- Calculate the average electronegativity, Σχ.
- Calculate the electronegativity difference, Δχ.
- Plot the point (Σχ, Δχ) on the diagram.
- Read the percentage ionic/covalent character from the diagram.
32.Which statement best explains why substance X is harder than substance Y?
Medium
- AThe atoms of different sizes in X disrupt the regular lattice, making it harder for layers to slide.
- BSubstance X has stronger covalent bonds between atoms.
- CSubstance Y has delocalised electrons that make it softer.
- DSubstance X has a higher melting point.
33.Substance X is a mixture of metals, not a compound.
Easy
True or false?
34.Silicon tetrachloride, SiCl4, is a simple molecular substance. How would its boiling point compare with that of an ionic substance of similar formula mass?
Hard
- ASiCl4 would have a lower boiling point because it has weak intermolecular forces.
- BSiCl4 would have a higher boiling point because it has strong covalent bonds.
- CSiCl4 would have a similar boiling point because both have similar formula masses.
- DSiCl4 would have a higher boiling point because it is covalent.
35.Which term describes a large molecule made by chemically linking many small molecules together?
Easy
- APolymer
- BMonomer
- CAlloy
- DCopolymer
36.What is the smallest number of repeating units a molecule must contain to be called a polymer?
Easy
- A50
- B2
- C10
- D100
37.Which of the following is a natural polymer?
Easy
- ADNA
- BNylon
- CPoly(ethene)
- DPVC
38.Which of the following best describes an alloy?
Easy
- AA mixture of metals, or a metal mixed with a non-metal such as carbon
- BA compound in which a metal is chemically bonded to a non-metal
- CA pure metal with a regular lattice of identical cations
- DA solution of a metal in water
39.Which statement about the bonding in a pure metal is correct?
Medium
- AMetallic bonds are non-directional, so atoms of different sizes can occupy the lattice
- BMetallic bonds are directional, so only atoms of identical size fit the lattice
- CMetal cations are held in fixed positions by covalent bonds
- DDelocalised electrons are localised between specific pairs of cations
40.A polymer must contain at least 50 repeating units.
Easy
True or false?
41.Complete the sentences about polymers using the correct terms.
Easy
Polymers are large molecules made by linking many small molecules called ____. A polymer must contain at least ____ repeating units.
42.Name the type of polymerisation in which monomers containing a C=C double bond join together with no by-product.
Medium
43.For caesium iodide, caesium has an electronegativity of 1.0 and iodine has an electronegativity of 2.7. Calculate the average electronegativity, Σχ, of caesium iodide.
Medium
44.Poly(ethene) is made by addition polymerisation of ethene. Calculate the percentage atom economy for this reaction.
Medium
45.Calculate the average electronegativity, Σχ, of aluminium oxide, Al2O3. Aluminium has an electronegativity of 1.6 and oxygen has an electronegativity of 3.4.
Medium
46.Calculate the electronegativity difference, Δχ, for aluminium oxide, Al2O3. Aluminium has an electronegativity of 1.6 and oxygen has an electronegativity of 3.4.
Medium
47.Calculate the electronegativity difference, Δχ, for silicon dioxide, SiO2. Silicon has an electronegativity of 1.9 and oxygen has an electronegativity of 3.4.
Medium
48.Calculate the electronegativity difference, Δχ, for caesium iodide. Caesium has an electronegativity of 1.0 and iodine has an electronegativity of 2.7.
Easy
49.Calculate the average electronegativity, Σχ, of aluminium chloride, AlCl3. Aluminium has an electronegativity of 1.6 and chlorine has an electronegativity of 3.2.
Medium
50.Calculate the electronegativity difference, Δχ, of aluminium chloride, AlCl3. Aluminium has an electronegativity of 1.6 and chlorine has an electronegativity of 3.2.
Medium
51.Calculate the electronegativity difference, Δχ, of aluminium oxide, Al2O3. Aluminium has an electronegativity of 1.6 and oxygen has an electronegativity of 3.4.
Medium
52.Calculate the average electronegativity, Σχ, of ammonia, NH3. Nitrogen has an electronegativity of 3.0 and hydrogen has an electronegativity of 2.2.
Medium
53.State why elements are found at the bottom of a triangular bonding diagram.
Medium
54.Suggest the position of ionic and covalent materials within a triangular bonding diagram. Explain your answer.
Medium
55.Name the x-axis and y-axis on a standard triangular bonding diagram.
Medium
56.Using Sections 9 and 17 of the Data Booklet, plot nitrogen trifluoride (NF3) on the triangular bonding diagram.
Medium
57.Explain why poly(2-chloropropene) is a strong material.
Hard
58.For caesium iodide, caesium has an electronegativity of 1.0 and iodine has an electronegativity of 2.7. Calculate the average electronegativity, Σχ, of caesium iodide.
Medium
- A3.85
- B2.85
- C1.85
- D0.85
59.Poly(ethene) is made by addition polymerisation of ethene. Calculate the percentage atom economy for this reaction.
Medium
- A100 %
- B120 %
- C90 %
- D110 %
60.Calculate the average electronegativity, Σχ, of aluminium oxide, Al2O3. Aluminium has an electronegativity of 1.6 and oxygen has an electronegativity of 3.4.
Medium
- A2.5
- B4.5
- C1.5
- D3.5
61.Calculate the electronegativity difference, Δχ, for aluminium oxide, Al2O3. Aluminium has an electronegativity of 1.6 and oxygen has an electronegativity of 3.4.
Medium
- A1.8
- B2.8
- C3.8
- D0.8
62.Calculate the electronegativity difference, Δχ, for silicon dioxide, SiO2. Silicon has an electronegativity of 1.9 and oxygen has an electronegativity of 3.4.
Medium
- A2.5
- B0.5
- C3.5
- D1.5
63.Calculate the electronegativity difference, Δχ, for caesium iodide. Caesium has an electronegativity of 1.0 and iodine has an electronegativity of 2.7.
Easy
- A0.7
- B2.7
- C3.7
- D1.7
64.Calculate the average electronegativity, Σχ, of aluminium chloride, AlCl3. Aluminium has an electronegativity of 1.6 and chlorine has an electronegativity of 3.2.
Medium
- A4.4
- B3.4
- C2.4
- D1.4
65.Calculate the electronegativity difference, Δχ, of aluminium chloride, AlCl3. Aluminium has an electronegativity of 1.6 and chlorine has an electronegativity of 3.2.
Medium
- A0.6
- B1.6
- C2.6
- D3.6
66.Calculate the electronegativity difference, Δχ, of aluminium oxide, Al2O3. Aluminium has an electronegativity of 1.6 and oxygen has an electronegativity of 3.4.
Medium
- A0.8
- B2.8
- C3.8
- D1.8
67.Calculate the average electronegativity, Σχ, of ammonia, NH3. Nitrogen has an electronegativity of 3.0 and hydrogen has an electronegativity of 2.2.
Medium
- A2.6
- B4.6
- C3.6
- D1.6

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