Bonding, structure, and the properties of matter

Section 4.2

18 spec leafs

Notes and three levels of exam-style practice for each registered specification leaf in this section.

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4.2.1.1

Chemical bonds

The three types of strong chemical bond are ionic, covalent and metallic: ionic bonding acts between oppositely charged ions, covalent bonding uses shared pairs of electrons, and metallic bonding involves delocalised electrons.
Use the elements present to choose a likely bonding type: metal with non-metal is usually ionic, non-metal with non-metal is covalent, and a metal element or alloy is metallic.
For example, sodium chloride forms by electron transfer, chlorine molecules contain shared electron pairs, and copper contains delocalised outer-shell electrons.
A common error is to name transferred or shared electrons without stating the electrostatic attraction that makes the bond strong.

Tier 1 · Easy

3 marks

ORIGINAL

State the type of strong chemical bonding in sodium chloride, oxygen and copper.

Tier 2 · Standard

4 marks

ORIGINAL

Magnesium reacts with chlorine to form magnesium chloride. Hydrogen atoms join to form hydrogen molecules. Compare how the strong bonds form in these two substances.

Tier 3 · Hard

6 marks

ORIGINAL

Substance P is a giant structure containing positive and negative particles. Substance Q is a giant structure containing one type of atom and mobile outer-shell electrons. Substance R contains separate groups of non-metal atoms joined by shared electron pairs. Identify the bonding in P, Q and R and explain the electrostatic attraction in each bond.

4.2.1.2

Ionic bonding

Ionic bonding begins when metal atoms lose outer-shell electrons to form positive ions and non-metal atoms gain those electrons to form negative ions.
For Groups 1 and 2, the positive ion charge matches the group number; Group 6 and 7 atoms form ions with charges $2-$ and $1-$ respectively.
For example, one calcium atom transfers one electron to each of two fluorine atoms, producing Ca2+ and two F− ions in CaF2.
In a dot-and-cross diagram, show only outer-shell electrons, use different symbols for their origins, and put each ion in brackets with its charge.

Tier 1 · Easy

2 marks

ORIGINAL

State the ions formed when potassium, a Group 1 metal, reacts with sulfur, a Group 6 non-metal.

Tier 2 · Standard

4 marks

ORIGINAL

Describe the electron transfer when magnesium chloride, MgCl2, forms, and state the charge on every ion produced from one magnesium atom.

Tier 3 · Hard

5 marks

ORIGINAL

Element X is in Group 2 and element Y is in Group 7. Describe a dot-and-cross diagram for the ionic compound they form and deduce its formula using X and Y.

4.2.1.3

Ionic compounds

An ionic compound is a giant lattice of positive and negative ions held by strong electrostatic attractions acting in all directions.
To find an empirical formula from a model, count each type of ion and simplify the ratio to the smallest whole numbers.
For example, a model containing $12$ X2+ ions and $24$ Y− ions has the ratio $1:2$ , so its empirical formula is XY2.
Dot-and-cross and ball-and-stick models are not literal pictures: they can misrepresent ion size, spacing, scale, bond direction and the full extent of the lattice.

Tier 1 · Easy

2 marks

ORIGINAL

Describe the structure and bonding in a solid ionic compound.

Tier 2 · Standard

2 marks

ORIGINAL

A model of an ionic lattice contains $18$ M2+ ions and $36$ N− ions. Work out the empirical formula of the compound.

Tier 3 · Hard

4 marks

ORIGINAL

A section of an ionic model contains $8$ A3+ ions and $12$ B2− ions. Deduce the empirical formula and give two limitations of using a ball-and-stick model for this lattice.

4.2.1.4

Covalent bonding

A covalent bond is a shared pair of electrons between atoms, and the bond itself is strong.
Recognise whether a diagram represents a small molecule, a long polymer chain or part of a giant covalent structure before interpreting its formula or bonds.
For example, methane has one carbon atom sharing one electron pair with each of four hydrogen atoms, giving CH4.
A line represents one covalent bond, not a physical stick; a small drawn portion may also hide the much larger extent of a polymer or giant structure.

Tier 1 · Easy

1 mark

ORIGINAL

A chlorine molecule contains two chlorine atoms joined by one covalent bond. State what this bond represents.

Tier 2 · Standard

4 marks

ORIGINAL

Describe a dot-and-cross diagram for one water molecule, H2O. Include the bonding pairs and the unshared outer-shell electrons on oxygen.

Tier 3 · Hard

3 marks

ORIGINAL

A model shows two nitrogen atoms joined together, with each nitrogen also joined to two hydrogen atoms. Deduce the molecular formula, count the covalent bonds shown, and give one limitation of the model.

4.2.1.5

Metallic bonding

Metals have giant structures in which atoms are arranged in a regular pattern and outer-shell electrons are delocalised throughout the structure.
When interpreting a metallic-bonding diagram, identify the regular positive metal ions and the delocalised electrons that can move through the whole lattice.
For example, a metallic-bonding model explains strength by the electrostatic attraction between each positive metal ion and the sea of negatively charged delocalised electrons throughout the lattice.
Do not describe a metal as separate molecules or say that its electrons belong to one neighbouring pair of atoms; the electrons are shared across the giant structure.

Tier 1 · Easy

2 marks

ORIGINAL

Name the two types of charged particle shown in the usual model of metallic bonding.

Tier 2 · Standard

3 marks

ORIGINAL

Explain why the bonding in a metal is strong.

Tier 3 · Hard

5 marks

ORIGINAL

A diagram shows identical circles in regular layers with many smaller negative symbols between them. A student says the diagram represents a simple molecule with electrons shared only between neighbouring pairs of atoms. Evaluate the student's statement.

4.2.2.1

The three states of matter

Solids, liquids and gases differ in particle arrangement and movement; melting, freezing, boiling and condensing are physical changes between these states.
To predict state, compare the temperature with the melting point and boiling point: below melting is solid, between them is liquid, and above boiling is gas.
For example, a substance with melting point $-20\,^{\circ}\text{C}$ and boiling point $70\,^{\circ}\text{C}$ is liquid at $25\,^{\circ}\text{C}$ .
Melting and boiling require energy to overcome forces between particles, whereas freezing and condensing transfer energy to the surroundings; individual particles do not themselves melt or boil. Higher tier only: the simple model omits forces and represents every particle as a solid, inelastic sphere.

Tier 1 · Easy

2 marks

ORIGINAL

At the boiling point, name the change from liquid to gas and the reverse change from gas to liquid.

Tier 2 · Standard

3 marks

ORIGINAL

Substance S has a melting point of $-15\,^{\circ}\text{C}$ and a boiling point of $84\,^{\circ}\text{C}$ . State its physical state at $-30\,^{\circ}\text{C}$ , $20\,^{\circ}\text{C}$ and $100\,^{\circ}\text{C}$ .

Tier 3 · Hard

5 marks

ORIGINAL

Substance P melts at $40\,^{\circ}\text{C}$ and boils at $85\,^{\circ}\text{C}$ . Substance Q melts at $780\,^{\circ}\text{C}$ and boils at $1400\,^{\circ}\text{C}$ . Compare their states at $100\,^{\circ}\text{C}$ and explain what the data suggest about the forces or bonds between their particles.

4.2.2.2

State symbols

Chemical equations use (s) for solid, (l) for liquid, (g) for gas and (aq) for a substance dissolved in water.
Choose each state symbol from the conditions and information given, then place it immediately after the correct chemical formula.
For example, magnesium reacting with hydrochloric acid can be written Mg(s) + 2HCl(aq) → MgCl2(aq) + H2(g).
Aqueous does not mean liquid: NaCl(aq) is sodium chloride dissolved in water, whereas NaCl(l) is molten sodium chloride.

Tier 1 · Easy

2 marks

ORIGINAL

State the meaning of the symbols (l) and (aq) in a chemical equation.

Tier 2 · Standard

3 marks

ORIGINAL

At room conditions, methane, oxygen and carbon dioxide are gases, while the water produced is liquid. Represent the complete combustion of methane by a balanced symbol equation with a state symbol after each formula.

Tier 3 · Hard

4 marks

ORIGINAL

A technician mixes solutions of barium chloride and sodium sulfate. The barium sulfate product is a solid precipitate, while sodium chloride stays dissolved. Give, in order, the state symbols for barium chloride, sodium sulfate, barium sulfate and sodium chloride.

4.2.2.3

Properties of ionic compounds

Ionic compounds contain strong electrostatic attractions in all directions through a regular giant lattice.
Explain high melting and boiling points by linking the many strong ionic bonds to the large energy transfer needed to overcome them.
For example, solid sodium chloride does not conduct because its ions are fixed, but molten NaCl conducts because its ions are free to move and carry charge.
Do not say that electrons move through an ionic compound or that every ionic compound conducts when solid.

Tier 1 · Easy

2 marks

ORIGINAL

State whether an ionic compound usually conducts electricity when solid and when molten.

Tier 2 · Standard

3 marks

ORIGINAL

Explain why magnesium oxide has a high melting point.

Tier 3 · Hard

6 marks

ORIGINAL

Compound T has a high melting point. It does not conduct electricity as a solid, but it conducts when molten and when dissolved in water. Explain all three observations and identify the likely structure of T.

4.2.2.4

Properties of small molecules

Substances made from small molecules are often gases or liquids with relatively low melting and boiling points because forces between molecules are weak.
When comparing molecular substances, a larger molecule generally has stronger intermolecular forces and therefore a higher melting or boiling point.
For example, boiling methane separates CH4 molecules by overcoming intermolecular forces; the strong covalent bonds within each molecule remain intact.
Small molecular substances usually do not conduct electricity because their molecules have no overall charge; do not explain low boiling points by calling covalent bonds weak.

Tier 1 · Easy

2 marks

ORIGINAL

Explain why a substance made of small molecules can have a low boiling point even though its covalent bonds are strong.

Tier 2 · Standard

3 marks

ORIGINAL

Molecule V is larger than molecule U. Both substances consist of small molecules. Predict which substance usually has the higher boiling point and explain your answer.

Tier 3 · Hard

5 marks

ORIGINAL

Substances A and B both contain neutral small molecules. A has relative molecular mass $34$ and boils at $22\,^{\circ}\text{C}$ ; B has relative molecular mass $122$ and boils at $168\,^{\circ}\text{C}$ . Explain the difference in boiling point and predict whether either pure substance conducts electricity.

4.2.2.5

Polymers

Polymers have very large molecules in which atoms are joined to other atoms by strong covalent bonds.
To recognise a polymer structure, trace a long molecular chain with a bonding pattern that repeats; separate chains are separate molecules.
The intermolecular forces between polymer molecules are relatively strong, so these polymer substances are solids at room temperature.
A polymer is not automatically a giant covalent structure: melting overcomes forces between its molecules, not the strong covalent bonds along each chain.

Tier 1 · Easy

2 marks

ORIGINAL

A substance consists of separate, very long carbon-based chains. Name this class of substance and state the type of bond joining atoms within each chain.

Tier 2 · Standard

3 marks

ORIGINAL

A short-chain molecular substance is a liquid at $20\,^\circ\text{C}$ , whereas material P, made from long covalent chains, is solid. Explain why P is solid at this temperature.

Tier 3 · Hard

4 marks

ORIGINAL

Material R contains long covalent chains with no covalent bonds from one chain to another. Material S is one continuous covalent network. Identify the polymer and explain two structural differences between R and S.

4.2.2.6

Giant covalent structures

A giant covalent structure is a continuous network in which all atoms are linked to other atoms by strong covalent bonds.
To identify one from a diagram, check that the bonding pattern continues throughout the structure instead of ending at separate molecules.
Melting or boiling requires many strong covalent bonds to be overcome, so giant covalent substances are solids with very high melting points.
Do not explain a giant covalent melting point using intermolecular forces: examples such as diamond and silicon dioxide do not consist of small, separate molecules.

Tier 1 · Easy

1 mark

ORIGINAL

A diagram shows atoms covalently bonded in a repeating network that extends in every direction. State the type of structure shown.

Tier 2 · Standard

3 marks

ORIGINAL

Silicon dioxide is solid at $1500\,^\circ\text{C}$ . Explain this observation using its structure and bonding.

Tier 3 · Hard

4 marks

ORIGINAL

Two covalent substances are heated. U melts at $84\,^\circ\text{C}$ and consists of separate molecules. V is still solid at $1700\,^\circ\text{C}$ and has no separate molecules. Deduce the structure of V and explain the difference in melting behaviour.

4.2.2.7

Properties of metals and alloys

Metals have giant structures of atoms held together by strong metallic bonding, so most metals have high melting and boiling points.
To explain why a pure metal can be shaped, link its regularly arranged layers of atoms to their ability to slide over one another.
Alloying introduces atoms of different sizes, distorting the regular layers and making it harder for the layers to slide, so the alloy is harder.
A common error is to say that alloys are harder because they contain more bonds; the required explanation is the distortion that obstructs layer movement.

Tier 1 · Easy

2 marks

ORIGINAL

Explain why a pure metal can be bent into shape.

Tier 2 · Standard

3 marks

ORIGINAL

A manufacturer replaces pure metal M with an alloy containing a second element whose atoms have a different size. Explain why the alloy is harder than M.

Tier 3 · Hard

5 marks

ORIGINAL

Pure metal Q bends when a force of $18\,\text{N}$ is applied, but an alloy of Q needs $47\,\text{N}$ . Both have high melting points. Explain both observations in terms of structure and bonding.

4.2.2.8

Metals as conductors

Metallic structures contain delocalised electrons that are not tied to one atom and can move through the structure.
For electrical conduction, identify the mobile delocalised electrons and state that they carry electrical charge through the metal.
For thermal conduction, energy is transferred rapidly through the structure by the delocalised electrons.
Do not credit the metal ions with moving through a solid wire: the ions stay in fixed positions while delocalised electrons move and transfer charge or energy.

Tier 1 · Easy

1 mark

ORIGINAL

Name the particles that carry electrical charge through a metal wire.

Tier 2 · Standard

3 marks

ORIGINAL

A metal strip completes a circuit, but a solid polymer strip does not. Explain why the metal conducts electricity.

Tier 3 · Hard

4 marks

ORIGINAL

Copper is used for both electrical wiring and the base of a saucepan. Explain how the same structural feature makes copper suitable for both uses.

4.2.3.1

Diamond

Diamond is a giant covalent structure in which each carbon atom forms four covalent bonds with other carbon atoms.
To explain a diamond property, begin with its four-bonded rigid network and then identify whether bond strength or electron mobility controls the property.
The many strong covalent bonds make diamond very hard and give it a very high melting point.
A common error is to assume diamond conducts merely because it is carbon; all four outer electrons of each atom are used in bonds, so there are no delocalised electrons.

Tier 1 · Easy

1 mark

ORIGINAL

In diamond, how many bonds per carbon?

Tier 2 · Standard

3 marks

ORIGINAL

Explain why a diamond-tipped cutting tool can scratch most materials.

Tier 3 · Hard

5 marks

ORIGINAL

Diamond is proposed for an electrically heated cutting element. Explain its high melting point and hardness, and decide whether it can carry the heating current.

4.2.3.2

Graphite

In graphite, each carbon atom forms three covalent bonds, producing layers of hexagonal rings with no covalent bonds between the layers.
Choose the relevant feature for each property: strong covalent bonds within layers affect melting, while the lack of covalent bonds between layers allows sliding.
One electron from each carbon atom is delocalised and can carry charge, so graphite conducts electricity in a similar way to metals.
Do not say graphite is soft because its covalent bonds are weak; its layers slide because only weak forces, not covalent bonds, act between them.

Tier 1 · Easy

1 mark

ORIGINAL

State the number of covalent bonds made by each carbon atom in graphite.

Tier 2 · Standard

3 marks

ORIGINAL

Explain why graphite can be used as an electrode.

Tier 3 · Hard

6 marks

ORIGINAL

Graphite is used in a high-temperature electrical contact that must also slide against another surface. Explain three properties that make graphite suitable.

4.2.3.3

Graphene and fullerenes

Graphene is a single layer of graphite: a one-atom-thick sheet of hexagonally arranged carbon atoms with delocalised electrons.
Relate graphene's strong covalent bonds, electrical conductivity and very small thickness to uses in composites and electronics.
Fullerenes are hollow carbon molecules based mainly on hexagonal rings, sometimes with five- or seven-membered rings; Buckminsterfullerene, C60, is spherical.
Do not confuse carbon nanotubes with solid rods: they are cylindrical fullerenes with very high length-to-diameter ratios and uses in nanotechnology, electronics and materials.

Tier 1 · Easy

1 mark

ORIGINAL

Describe the relationship between graphene and graphite.

Tier 2 · Standard

4 marks

ORIGINAL

An electronic sensor needs a conducting layer that adds very little thickness. Explain why graphene is suitable.

Tier 3 · Hard

5 marks

ORIGINAL

Three carbon materials are described: A is a one-atom-thick sheet; B is a hollow sphere containing 60 carbon atoms; C is a hollow cylinder whose length is far greater than its diameter. Identify A, B and C, then give one suitable type of application for A and one for C.

4.2.4.1

Sizes of particles and their properties (chemistry only)

Nanoscience concerns structures from $1$ to $100\,\text{nm}$ , of the order of a few hundred atoms; fine particles span $100$ to $2500\,\text{nm}$ and coarse particles span $2500$ to $10\,000\,\text{nm}$ .
For a cube of side $a$ , calculate surface area as $6a^2$ and volume as $a^3$ , then simplify the surface-area-to-volume ratio; it equals $6/a$ .
Reducing a cube's side by a factor of $10$ increases its surface-area-to-volume ratio by a factor of $10$ , so nanoparticles can behave differently from the bulk material.
A common error is to compare surface areas alone: particle effects depend on surface area relative to volume, and a higher ratio can make a smaller quantity effective.

Tier 1 · Easy

2 marks

ORIGINAL

Particle T is $72\,\text{nm}$ across. A typical atom is $0.24\,\text{nm}$ across. State whether T is nano, fine or coarse, and calculate how many atom diameters span T.

Tier 2 · Standard

3 marks

ORIGINAL

A nano-cube has edge length $5\,\text{nm}$ . Calculate its surface area, volume and surface-area-to-volume ratio.

Tier 3 · Hard

4 marks

ORIGINAL

Fine particles have side $800\,\text{nm}$ ; nano-sized cubes of the same material have side $40\,\text{nm}$ . Determine how many times larger the nano cubes' surface-area-to-volume ratio is. If effectiveness per gram is proportional to this ratio and $12\,\text{g}$ of the fine particles is effective, estimate the effective mass of nano cubes.

4.2.4.2

Uses of nanoparticles (chemistry only)

Nanoparticles are used in medicine, electronics, cosmetics and sun creams, deodorants and catalysts, with new applications still being researched.
To evaluate a proposed use, compare relevant benefits, such as a smaller effective quantity or useful surface behaviour, with evidence about cost, performance and risk.
For example, a nanoparticulate catalyst can expose a large surface area using little material, potentially reducing resource use while maintaining reaction performance.
Do not claim that nanoparticles are proven safe or harmful without evidence: their possible health and environmental risks must be considered because their small size may change how they interact with organisms.

Tier 1 · Easy

2 marks

ORIGINAL

Give two application areas in which nanoparticles are used.

Tier 2 · Standard

4 marks

ORIGINAL

A catalyst works equally well when a factory replaces $8.0\,\text{g}$ of bulk material with $0.40\,\text{g}$ of nanoparticles. Suggest two advantages and one possible disadvantage of the change.

Tier 3 · Hard

6 marks

ORIGINAL

A company compares two sun creams. Product J uses larger particles, blocks $96\%$ of ultraviolet radiation and leaves a visible layer. Product K uses nanoparticles, blocks $94\%$ , is transparent and needs one quarter as much active material. Tests detect K's particles inside $2\%$ of sampled skin cells, but no harm has been established. Evaluate which product the company should develop.