S2.4 From models to materials

1. Melting splits its MgO molecules into separate ions, which then carry the current. — A student who pictures ionic compounds as made of molecules picks this. Solid MgO contains no molecules; it is already a lattice of Mg²⁺ and O²⁻ ions. Melting does not create ions, it lets the existing ions move.
2. Its ions become liquid when it melts, and liquid ions can carry charge but solid ones cannot. — A student who gives single particles the properties of the bulk material picks this. An ion is not solid or liquid; 'liquid' describes how the particles are arranged. In the melt the same ions are no longer held in fixed positions, so they can move.
3. Its ions cannot move in the solid, but in the melt they move and carry the current. — High in the triangle, the ionic model applies: MgO is a lattice of Mg²⁺ and O²⁻ ions. In the solid the ions cannot move, so no current flows; when it melts the ions become mobile and carry the current.
4. Melting sets electrons free, which then move through the liquid and carry the current. — A student who thinks current is always carried by electrons picks this. MgO has no delocalized electrons, solid or molten; in the melt the current is carried by Mg²⁺ and O²⁻ ions moving to the electrodes.

Syllabus statement S2.4.1 · Read this in Learn

1. 2.8 — A student who averages over every atom in the formula gets (1.6 + 3 × 3.2)/4 = 2.8. The x-coordinate is the mean of the two elements' electronegativities, (1.6 + 3.2)/2 = 2.4; the number of chlorine atoms does not enter.
2. 4.8 — A student who adds the electronegativities without dividing by 2 gets 4.8. The x-coordinate is an average, (1.6 + 3.2)/2 = 2.4; 4.8 is off the scale, since no element has χ above 4.0.
3. 1.6 — A student who interchanges the axes gives Δχ = 3.2 − 1.6 = 1.6 as the x-coordinate. The bottom vertices have Δχ = 0, so Δχ (1.6) is the y-coordinate, which measures ionic character; the x-axis is the average electronegativity (2.4).
4. 2.4 — Both bottom vertices have Δχ = 0 and the ionic vertex at the top has the largest Δχ, so Δχ is the y-coordinate and x is the average electronegativity of the two elements: (1.6 + 3.2)/2 = 2.4. Each element is counted once, whatever the formula.

Working The bottom edge (both bottom vertices) has Δχ = 0 and the ionic vertex at the top has the largest Δχ, so Δχ is the vertical (y) coordinate and the horizontal (x) coordinate is the average electronegativity. x = (χ(Al) + χ(Cl))/2 = (1.6 + 3.2)/2 = 2.4 (no units). The y-coordinate, Δχ = 3.2 − 1.6 = 1.6, is not asked.

Syllabus statement S2.4.2 · Read this in Learn

1. A compound of a metal with other elements, bonded in a fixed ratio by formula — A student who thinks the metals react to form a compound picks this. An alloy is a mixture with variable composition and no fixed formula; the atoms of the added element sit in the metallic lattice.
2. A mixture of a metal with other metals or non-metals, in variable amounts — An alloy is a mixture, so its composition can vary, and the added element can be a metal (zinc in brass) or a non-metal (carbon in steel). Alloys usually have enhanced properties, such as greater hardness.
3. A mixture of two or more metals, with no atoms of a non-metal present in it — A student who learned the definition 'a mixture of metals' picks this. Alloys can contain non-metals: steel is iron with a small percentage of carbon.
4. A mixture of a metal with another element, made so that it is softer — A student who thinks mixing weakens a metal picks this. Alloys are usually harder and stronger than the pure metal, because the added atoms disrupt the layers so they slide less easily.

Syllabus statement S2.4.3 · Read this in Learn

1. Polymers are synthetic plastics; substances made by living things are not. — A student who equates 'polymer' with 'plastic' picks this. Many polymers are natural: cellulose, starch, proteins, DNA and natural rubber are all macromolecules made of repeating monomer units.
2. Fats are natural polymers made of many glycerol and fatty acid units. — A student who assumes every large biological molecule is a polymer picks this. A fat (triglyceride) contains one glycerol and three fatty acid units; it has no long chain of repeating units, so it is not a polymer.
3. Glucose is a natural polymer, as it is the building block of starch and cellulose. — A student who confuses monomer with polymer picks this. Glucose is the monomer; starch and cellulose are the polymers made from many glucose units.
4. Cellulose (natural) and nylon (synthetic) are both polymers built from monomers. — Cellulose is a natural polymer of glucose, made by plants; nylon is a synthetic polyamide made industrially. Both are macromolecules built from many repeating monomer units, so being a polymer does not depend on being natural or synthetic.

Syllabus statement S2.4.4 · Read this in Learn

1. –CH₂=CH(CH₃)–, keeping each monomer's C=C — A student who thinks the monomer's double bond survives picks this. Polymerization happens because one bond of each C=C breaks; if the C=C remained, each carbon here would have five bonds.
2. –CH₂–CH(CH₃)–, one unit of a single long chain — One bond of each C=C breaks and the two former C=C carbons form the main chain, with CH₃ as a side group. The formula [–CH₂–CH(CH₃)–]ₙ represents one chain molecule containing n of these units.
3. –CH₂–CH₂–CH₂–, all three carbons in the main chain — A student who puts every carbon of the monomer into the chain picks this. Only the two C=C carbons form the main chain; the CH₃ group becomes a side group, so the chain has CH₃ branches on alternate carbons.
4. CH₃CH₂CH₃, one of n separate small molecules — A student who reads n as a number of separate molecules writes the unit as a complete molecule, CH₃CH₂CH₃ (propane). The repeating unit has open bonds at both ends because each unit is covalently bonded to the next in one long chain, [–CH₂–CH(CH₃)–]ₙ; propane has no C=C and cannot polymerize.

Syllabus statement S2.4.5 · Read this in Learn

1. The bonds are ionic up to AlCl₃ and covalent from SiCl₄ on, where the metals end. — A student who decides bond type by metal/non-metal class picks this. The Δχ values show no break at aluminium: the step from AlCl₃ (1.6) to SiCl₄ (1.3) is no larger than the others. AlCl₃ already has substantial covalent character (it sublimes at about 180 °C as Al₂Cl₆ molecules), and SiCl₄ still has polar bonds.
2. Ionic character decreases and covalent character increases gradually along the series. — Δχ falls in small steps (2.3, 1.9, 1.6, 1.3, 1.0), so each compound lies a little lower in the bonding triangle than the one before and a little further towards the covalent side. The bonding changes by degrees: NaCl is mainly ionic, PCl₃ mainly covalent, and AlCl₃ lies between them.
3. Ionic character increases along the series, as the average electronegativity rises. — A student who treats average electronegativity as the measure of ionic character picks this. The average does rise (2.05, 2.25, 2.4, 2.55, 2.7), but that moves compounds towards the covalent side. Ionic character is measured by Δχ, which falls along the series.
4. In SiCl₄ some of the Si–Cl bonds are ionic and the others are fully covalent. — A student who reads intermediate character as a mixture of bond types picks this. The four Si–Cl bonds in SiCl₄ are equivalent: each is a polar covalent bond with the same partial ionic character (Δχ = 1.3).

Syllabus statement S2.4.1 · Read this in Learn

1. It has a low boiling point, as its molecules are held by weak intermolecular forces. — With Δχ = 1.2 and a high average, SnCl₄ lies low in the triangle towards the covalent vertex, far below NaCl. It is predicted to be molecular; indeed it is a liquid that boils at 114 °C, because only weak London forces hold its tetrahedral SnCl₄ molecules together.
2. It has a high melting point, as a metal chloride is a lattice of metal and chloride ions. — A student who classifies every metal–non-metal compound as ionic picks this. The electronegativities, not the element classes, decide the position: Δχ = 1.2 is small, so SnCl₄ has mainly covalent character and is a molecular liquid at room temperature.
3. Its vapour consists of separate Sn and Cl atoms, as boiling must break its Sn–Cl bonds. — A student who thinks boiling breaks covalent bonds picks this. Boiling a molecular substance separates whole SnCl₄ molecules by overcoming the intermolecular forces between them; the Sn–Cl bonds stay intact, so the vapour consists of SnCl₄ molecules and the boiling point is low (114 °C).
4. It conducts electricity when solid, as it contains atoms of the metal tin. — A student who expects a compound to behave like the metal it contains picks this. SnCl₄ has no delocalized electrons and no ions; its electrons are in Sn–Cl bonds within molecules, so it does not conduct in any state.

Syllabus statement S2.4.2 · Read this in Learn

1. Bronze is harder, as the larger tin atoms disrupt the layers of copper atoms. — In pure copper the layers of identical cations slide over each other easily. The larger tin atoms distort the regular layers, so the layers cannot slide as easily and bronze is harder. The non-directional metallic bonding holds the tin atoms in the lattice, so bronze remains a metal.
2. Bronze is softer, as a mixture of metals is weaker than a pure metal. — A student who thinks a mixture must be weaker than a pure substance picks this. Alloying normally makes a metal harder: bronze is harder than copper, which is why it was used for tools and weapons.
3. Bronze is softer, as its tin atoms are themselves soft and pass this on to the alloy. — A student who gives atoms the properties of the bulk metal picks this. A tin atom is not soft or hard; softness is a property of pure tin as a material. In bronze, the tin atoms make the copper harder by disrupting its layers.
4. Bronze is harder, as copper and tin form a compound with a fixed formula. — A student who thinks an alloy is a compound picks this. Bronze is a mixture whose tin content can vary; its hardness comes from different-sized atoms disrupting the layers, not from a new compound.

Syllabus statement S2.4.3 · Read this in Learn

1. The chains gain enough energy to overcome the forces between them and slide past each other. — Poly(ethene) consists of long chains held together only by intermolecular (London) forces. Heating gives the chains enough energy to overcome these forces partly, so they slide past each other and the plastic can be reshaped. The covalent bonds in the chains are not broken.
2. Heat flows into the gaps between the chains as a substance and pushes the chains apart. — A student who pictures heat as a substance picks this. Heat is energy transferred to the plastic, not a material that fills the gaps; the chains gain kinetic energy, partly overcome the intermolecular forces between them and slide past each other.
3. The covalent bonds between neighbouring chains break, so the chains separate. — A student who pictures the chains as covalently bonded to each other picks this. In poly(ethene) there are no covalent bonds between chains; the chains are held together by intermolecular forces.
4. Each chain molecule itself becomes soft, so the plastic as a whole softens. — A student who gives molecules the properties of the bulk material picks this. Softness is a property of the material; it softens because the chains can move past each other more easily, not because each molecule becomes soft.

Syllabus statement S2.4.4 · Read this in Learn

1. It is a molecular substance like CS₂, as its small Δχ means shared electron pairs. — A student who uses Δχ alone picks this. A small Δχ places a substance near the bottom edge, but the average electronegativity decides where: CS₂ (average 2.6) is at the covalent side, P (average 1.15) at the metallic side.
2. It conducts when molten but not as a solid, as its unequal χ values mean it has ions. — A student who thinks any electronegativity difference creates ions picks this. A Δχ of 0.3 is far too small for electron transfer; P lies near the bottom of the triangle, close to the metallic vertex, not near the ionic vertex at the top.
3. Some of its bonds are ionic and the rest are metallic, as its Δχ is small but not zero. — A student who reads intermediate character as a mixture of bond types picks this. The bonding in P has a single character throughout: with Δχ = 0.3 and a low average electronegativity, P lies just above the bottom edge near the metallic vertex, so it is metallic bonding with slight polarity, not a mixture of separate ionic and metallic bonds.
4. It conducts electricity as a solid, as its valence electrons are delocalized. — P and CS₂ both lie on or near the bottom edge, but P has a low average electronegativity and so lies near the metallic vertex. Atoms with low electronegativity hold their valence electrons weakly, so these electrons are delocalized, and P is predicted to conduct as a solid, like a metal.

Syllabus statement S2.4.2 · Read this in Learn

1. Copper and zinc atoms bond in a fixed ratio, as they would in a compound of the two. — A student who thinks an alloy is a compound picks this. Brasses contain from about 5% to 45% zinc; there is no fixed ratio, because the bonding is non-directional metallic bonding, not bonding between particular pairs of atoms.
2. Covalent bonds join the atoms, as metallic bonding needs atoms of a single element. — A student who confines metallic bonding to pure elements picks this. Directional covalent bonds would give no mobile electrons, so brass could not conduct. Brass keeps the metallic bonding of copper.
3. Delocalized electrons hold all the cations, as the bonding is non-directional. — Brass is held by metallic bonding, which is non-directional: the delocalized electrons attract every cation, whatever its element. So zinc atoms fit into the lattice, the delocalized electrons carry current, and layers can be moved without breaking the bonding.
4. Zinc atoms give electrons to copper atoms, so that ionic bonds form between them. — A student who thinks any difference in electronegativity leads to electron transfer picks this. Copper (χ 1.9) and zinc (χ 1.7) differ very little, so no ions of opposite charge form; an ionic solid would also be brittle and would not conduct when solid.

Syllabus statement S2.4.3 · Read this in Learn

1. It is an electrical insulator, as it contains no metal atoms to carry a current. — A student who decides conductivity from the elements present picks this. Conductivity depends on mobile charged particles, not on metal atoms: graphite contains no metal and conducts, and magnesium oxide contains a metal but does not conduct as a solid.
2. It softens on heating, as the covalent bonds in its chains break apart. — A student who thinks softening breaks covalent bonds picks this. Heating overcomes the intermolecular forces between chains; the C–C and C–H bonds within the chains stay intact.
3. It is an electrical insulator, as all of its valence electrons are in localized bonds. — Poly(ethene) consists of chains of C–C and C–H covalent bonds. Every valence electron is held in a bond between two particular atoms, and there are no ions, so there are no mobile charged particles to carry a current.
4. It does not biodegrade, as synthetic substances cannot be broken down by organisms. — A student who thinks being synthetic is the reason picks this. Poly(ethene) persists because its long chains of strong, non-polar C–C and C–H bonds are not broken down by microorganisms; some synthetic polymers, such as poly(lactic acid), do biodegrade.

Syllabus statement S2.4.4 · Read this in Learn

1. (CH₃)₃CH — A student who thinks the bonds do not change on polymerization gives a monomer with the same single bonds as the chain. An alkane has no C=C to break, so it cannot form an addition polymer; the monomer must be the alkene (CH₃)₂C=CH₂.
2. CH₂=CHCH₂CH₃ — A student who puts every carbon into the main chain reads the four carbons of each unit as a straight chain. The CH₃ groups are side groups on one main-chain carbon, so the monomer is branched: (CH₃)₂C=CH₂. But-1-ene would give –CH₂–CH(CH₂CH₃)–.
3. (CH₃)₃COH — A student who thinks addition polymerization releases water adds H₂O to the repeating unit. No small molecule is lost in addition polymerization; the monomer has the same atoms as the repeating unit, C₄H₈.
4. (CH₃)₂C=CH₂ — The repeating unit is –CH₂–C(CH₃)₂–: a two-carbon unit of the main chain with two CH₃ side groups on one carbon. Restoring the C=C between the two main-chain carbons gives (CH₃)₂C=CH₂. The monomer has exactly the same atoms as the repeating unit.

Syllabus statement S2.4.5 · Read this in Learn