Bonding, structure, and the properties of matter

Explanation

• The three types of strong chemical bond are ionic, covalent and metallic. Ionic bonding is the electrostatic attraction between oppositely charged ions, usually formed when electrons transfer from metal atoms to non-metal atoms.
• A covalent bond is a shared pair of electrons between non-metal atoms.
• Metallic bonding is the strong electrostatic attraction between positive metal ions and delocalised electrons throughout a giant structure.
• Use the elements and particles shown to identify the bonding: metal plus non-metal suggests ionic; non-metals joined in molecules or networks are covalent; and a metal element or alloy is metallic.
• Examiners expect both the correct particles and the attraction between them, not only a bond name.

Explanation

• Ionic compounds form when electrons are transferred so that atoms gain the electronic structures of noble gases. Metal atoms lose outer-shell electrons and become positive ions; non-metal atoms gain electrons and become negative ions.
• Group 1 and Group 2 metals form $1+$ and $2+$ ions, while Group 6 and Group 7 non-metals form $2-$ and $1-$ ions.
• The compound must be electrically neutral, so the total positive and negative charges balance.
• In a dot-and-cross diagram, show outer-shell electrons only, distinguish their origins, enclose each ion in square brackets and write its charge.
• The ionic bond is then the electrostatic attraction between the resulting oppositely charged ions.

Explanation

• An ionic compound has a regular giant lattice containing positive and negative ions.
• Strong electrostatic forces of attraction act in all directions between oppositely charged ions; it is not a collection of separate molecules.
• Ionic structures must be recognised from dot-and-cross, ball-and-stick and two- or three-dimensional diagrams.
• To deduce an empirical formula from a model, count each ion type and simplify the ratio to the smallest whole numbers, checking that the total charge balances.
• Models are useful representations but not literal pictures: they may show incorrect relative sizes and separations, suggest directional sticks, omit charges or display only a tiny section of the repeating giant lattice.

Explanation

• A covalent bond is a shared pair of electrons between atoms, and bonds within molecules or giant structures are strong.
• Covalent substances may consist of small molecules, very large polymer molecules or giant covalent structures such as diamond and silicon dioxide.
• Required skills include recognising these structures from formulae and bonding diagrams, drawing dot-and-cross diagrams for the specified simple molecules, and using one line to represent each single covalent bond.
• Counting atoms in a complete molecular model gives its molecular formula.
• Diagrams are simplified: lines are not physical sticks, atom sizes and bond angles may be unrealistic, and a small section may represent a much larger polymer or giant structure.

Explanation

• Metals consist of giant structures of atoms arranged in a regular pattern. Their outer-shell electrons become delocalised, so they are free to move through the whole structure rather than belonging to one atom or one pair of atoms.
• The remaining positive metal ions form a regular lattice. Strong metallic bonding arises from the electrostatic attraction between these positive ions and the negatively charged delocalised electrons.
• A diagram should be interpreted by identifying both components and the repeated giant arrangement.
• Metals must not be described as separate molecules or as positive ions attracting each other.
• Examiners reuse this model in explanations of conductivity and the strength of metals.

Explanation

• The three states of matter are solid, liquid and gas. Melting and freezing occur at the melting point; boiling and condensing occur at the boiling point.
• To predict state, compare the given temperature with both change-of-state temperatures: below the melting point the substance is solid, between the points it is liquid, and above the boiling point it is a gas.
• Changing state transfers energy because forces between particles must be overcome or form; stronger forces give higher melting and boiling points.
• Atoms do not themselves have bulk properties such as being solid.
• Higher tier: the simple particle model omits forces and represents every particle as a solid, inelastic sphere.

Explanation

• State symbols show the physical state of each substance in a chemical equation: (s) means solid, (l) means liquid, (g) means gas and (aq) means dissolved in water.
• Place each symbol immediately after its formula and use the conditions or information in the question rather than guessing from the substance name.
• A solution is aqueous, while a precipitate is solid and a gas released during a reaction is gaseous.
• Aqueous does not mean the same as liquid: NaCl(aq) is sodium chloride dissolved in water, whereas NaCl(l) is molten sodium chloride.
• Examiners may require suitable state symbols as part of a complete balanced equation.

Explanation

• Ionic compounds have regular giant lattices with strong electrostatic forces of attraction in all directions between oppositely charged ions. Their melting and boiling points are high because large amounts of energy are needed to overcome the many strong ionic bonds.
• Electrical conductivity depends on whether charged particles can move.
• In a solid ionic compound, the ions are held in fixed lattice positions, so charge cannot flow.
• When the compound is molten or dissolved in water, its ions are free to move and carry charge, so it conducts.
• Examiners expect a full structure–bonding–property chain and the correct charge carrier: ions move, not electrons.

Explanation

• Substances made of small molecules are usually gases or liquids with relatively low melting and boiling points. The covalent bonds within each molecule are strong, but the intermolecular forces between separate molecules are weak.
• Melting or boiling overcomes these intermolecular forces; it does not break the covalent bonds.
• Intermolecular forces increase with molecular size, so larger molecules generally require more energy to separate and have higher melting and boiling points.
• Small molecular substances do not conduct electricity because their molecules have no overall electric charge.
• In explanations, distinguish clearly between forces between molecules and bonds within molecules, then link the relevant attraction to the observed bulk property.

Explanation

• Polymers have very large molecules. Within each polymer molecule, atoms are linked by strong covalent bonds in a long repeating chain.
• Separate chains remain separate molecules and are held near one another by intermolecular forces. These forces are relatively strong compared with those between small molecules, so polymer substances are solids at room temperature.
• A polymer diagram can be recognised by tracing a repeated bonding pattern along a long chain.
• It must not be confused with a giant covalent structure: a polymer contains separate very large molecules, whereas covalent bonds extend throughout a giant network.
• Melting a polymer overcomes intermolecular forces between chains, not the covalent bonds along each chain.

Explanation

• A giant covalent structure is a continuous network in which all atoms are linked to other atoms by strong covalent bonds.
• Diamond, graphite and silicon dioxide are examples.
• A diagram shows only part of the structure, so the key feature is that the bonding pattern continues rather than ending at the boundary of a separate molecule.
• Giant covalent substances are solids with very high melting points because many strong covalent bonds must be overcome to melt or boil them, requiring a large energy transfer.
• Intermolecular forces must not be used in this explanation because a giant covalent substance does not consist of small, separate molecules.

Explanation

• Metals have giant structures of atoms with strong metallic bonding, so most have high melting and boiling points. In a pure metal, equal-sized atoms form regular layers.
• These layers can slide over one another when a force is applied, allowing the metal to be bent and shaped.
• Pure metals are too soft for many uses, so they are mixed with other elements to make alloys.
• Atoms of different sizes distort the regular layers and make it harder for them to slide, so the alloy is harder.
• The required explanation is not that an alloy has more bonds; it is the link between different atom sizes, layer distortion and reduced movement.

Explanation

• Metallic structures contain delocalised electrons that are free to move through the whole giant structure. These electrons explain both electrical and thermal conduction.
• During electrical conduction, mobile delocalised electrons carry electrical charge through the metal. During thermal conduction, the delocalised electrons transfer energy through the structure.
• The positive metal ions remain in fixed lattice positions in a solid wire and are not the moving charge carriers.
• An exam answer must identify the delocalised electrons, state that they can move, and link their movement to the property being explained.
• The word ‘electrons’ alone does not establish why charge or energy can pass through the material.

Explanation

• Diamond is a giant covalent structure in which every carbon atom forms four strong covalent bonds with other carbon atoms. This produces a rigid network extending throughout the structure.
• Many strong covalent bonds must be overcome to melt diamond, so it has a very high melting point.
• The rigid arrangement also makes diamond very hard.
• Diamond does not conduct electricity because all four outer electrons from each carbon atom are used in covalent bonds, leaving no delocalised electrons free to move and carry charge.
• Property explanations must select the relevant structural feature: bond strength and network rigidity explain melting point and hardness, while absence of mobile electrons explains conductivity.

Explanation

• In graphite, each carbon atom forms three strong covalent bonds with other carbon atoms, creating layers of hexagonal rings. There are no covalent bonds between adjacent layers, so weak forces between the layers allow them to slide over one another.
• This makes graphite soft and useful where layers must move. Strong covalent bonds within the layers require much energy to overcome, giving graphite a high melting point.
• The fourth outer electron from each carbon atom is delocalised.
• These electrons can move through the structure and carry charge, so graphite conducts electricity in a similar way to metals.
• Explanations must keep bonding within layers separate from forces between layers.

Explanation

• Graphene is a single layer of graphite: a one-atom-thick sheet of carbon atoms joined in hexagonal rings. Its strong covalent bonds, delocalised electrons and very small thickness make it useful in electronics and composites.
• Fullerenes are hollow molecules made only of carbon. Their structures are based mainly on hexagonal rings but may also contain rings of five or seven carbon atoms.
• Buckminsterfullerene, C$_{60}$, is spherical. Carbon nanotubes are cylindrical fullerenes with very high length-to-diameter ratios.
• Their properties support uses in nanotechnology, electronics and materials.
• Diagrams and descriptions may require graphene, spherical fullerenes and nanotubes to be distinguished by shape and bonding.

Explanation

• Nanoscience concerns structures from $1$ to $100\,\mathrm{nm}$, of the order of a few hundred atoms. Fine particles have diameters from $100$ to $2500\,\mathrm{nm}$, while coarse particles span $2500$ to $10\,000\,\mathrm{nm}$.
• Nano dimensions may be compared with typical atomic and molecular dimensions using ratios and standard form.
• For a cube of side $a$, surface area is $6a^2$, volume is $a^3$, and the surface-area-to-volume ratio is $6/a$.
• Reducing the side by a factor of $10$ increases this ratio by a factor of $10$.
• A high ratio can give nanoparticles different properties from the bulk material and make smaller quantities effective.

Explanation

• Nanoparticles have applications in medicine, electronics, cosmetics and sun creams, deodorants and catalysts, and new uses remain an active area of research. Their high surface-area-to-volume ratio can make a smaller quantity effective, which may reduce material use, cost or waste.
• Evaluation questions provide information about a specified application and require advantages to be weighed against disadvantages.
• Possible health and environmental risks must be considered because the small particles may interact with organisms differently from bulk material.
• Evidence must control the conclusion: finding that particles enter cells does not by itself prove harm, while absence of proven harm does not prove safety.
• A justified judgement should compare performance, quantity, cost and uncertainty.