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O-Level Chemical Bonding and Structure
ionic, covalent and metallic bonding and how structure shapes a material's properties
What the syllabus expects
- Explain how ions form through the loss or gain of electrons, noting that the resulting ions usually end up with a noble gas electron arrangement.
- Show, using 'dot-and-cross' diagrams, how ionic bonds form between metals and non-metals, for example NaCl and MgCl2.
- State that ionic substances form a giant lattice in which electrostatic attraction holds the ions together, for example NaCl.
Scope: Drawing ionic lattice diagrams is not required. - Connect the physical characteristics of ionic compounds, including how they conduct electricity, to their lattice arrangement.
- Explain that a covalent bond forms when a pair of electrons is shared, leaving the bonded atoms in the molecule with a noble gas electron arrangement in most cases.
- Use 'dot-and-cross' diagrams to show covalent bonding between non-metal atoms, for example H2, O2, H2O, CH4 and CO2.
- Work out how electrons are arranged in further covalent molecules.
- Link the physical behaviour of covalent substances, conductivity included, to their bonding and structure.
- Describe a metal as a lattice of positive ions surrounded by a 'sea of electrons'.
- Account, in terms of structure, for the usual physical properties of metals as solids: high melting and boiling points, malleability and good conduction of heat and electricity.
- Distinguish between elements, compounds and mixtures.
- Define an alloy as a metal blended with another element, such as brass or stainless steel.
- Recognise metals and alloys when shown structural diagrams.
- Give reasons why an alloy's physical properties differ from those of the elements it is made from.
- Compare the structures of simple molecular substances (such as iodine and methane), macromolecules (such as poly(ethene)) and giant covalent materials (such as diamond, graphite and silicon dioxide/sand) in order to deduce their properties.
- Contrast how diamond and graphite are bonded and structured in order to deduce properties such as electrical conductivity and their lubricating or cutting action.
Scope: Drawing these structures is not required. - Predict a substance's chemical and physical properties from its bonding and structure, and work in the reverse direction as well.
How it's examined
Questions on this topic most often ask you to explain, show. About 15% of the past-paper style questions in Rae's bank for this subject sit in this topic.
Worked examples
Example 1 (3 marks)
Explain why substance X conducts electricity differently as a solid compared with as a liquid.
Show the worked answer
X is an ionic compound made of oppositely charged ions. In the solid, the ions are held in fixed positions in the giant ionic lattice by strong electrostatic forces and cannot move, so there are no mobile charge carriers and the solid does not conduct. When melted (liquid), the lattice breaks down and the ions become free to move and carry charge, so the liquid conducts electricity.
Example 2 (2 marks)
Predict whether borospherene is likely to have a high or a low boiling point, and give reasons for your answer.
Show the worked answer
Borospherene is a simple molecular (discrete cluster) substance, analogous to fullerene molecules. Its molecules are held to one another only by weak intermolecular (van der Waals) forces of attraction, not by a giant covalent network. Only a small amount of energy is needed to overcome these weak intermolecular forces, so it will have a LOW boiling point.
Example 3 (2 marks)
Predict whether borospherene will boil at a higher or a lower temperature than methane, CH4, and give the reason for your prediction.
Show the worked answer
Both borospherene and methane are simple molecular (simple covalent) substances held together by weak intermolecular forces. Borospherene is a much larger molecule (B40) with far more electrons than CH4, so it has stronger intermolecular (van der Waals / dispersion) forces of attraction between its molecules. More energy is needed to overcome these forces, so it boils at a higher temperature than methane.
More worked questions on this topic
- When ammonium chloride breaks down, hydrogen chloride gas is produced. Using a dot-and-cross di (2 marks)
- When sodium is burned in air it produces sodium oxide, as illustrated in the figure. Once the r (2 marks)
- In its compounds sulfur can hold up to 12 electrons in its outer shell; two such compounds are (2 marks)
More O-Level Pure Chemistry topics
Experimental Chemistry · The Particulate Nature of Matter · Chemical Calculations · Acid-Base Chemistry · Qualitative Analysis · Redox Chemistry · all of O-Level Pure Chemistry