How Bonding And Structure Are Related To The Properties Of Substances
Resumen
Sodium is a soft metal you can cut with a knife. Chlorine is a choking green gas. Push them together and you get a hard, colourless crystal that has to be heated past 800 °C before it will even melt, and that you sprinkle on your food. Nothing about the two starting elements predicts that. What changed was not the atoms but the way they are held together and the shape of the structure they build.
This lesson is about reading a substance backwards: you are handed a melting point, a boiling point and a note about whether it conducts electricity, and you work out what kind of structure must be behind those numbers. You will meet three structural families, learn the one comparison that examiners test more than any other, and practise the sentence patterns that turn a correct idea into full marks.
Objetivos
- Ionic compounds have regular structures (giant ionic lattices) in which there are strong electrostatic forces of attraction in all directions between oppositely charged ions. These compounds have high melting points and high boiling points because of the large amounts of energy needed to break the many strong bonds.
- When melted or dissolved in water, ionic compounds conduct electricity because the ions are free to move and carry the current. Knowledge of the structures of specific ionic compounds other than sodium chloride is not required.
- Substances that consist of simple molecules are gases, liquids or solids that have relatively low melting points and boiling points.
- Substances that consist of simple molecules have only weak forces between the molecules (intermolecular forces). It is these intermolecular forces that are overcome, not the covalent bonds, when the substance melts or boils. Students should understand that intermolecular forces are weak compared with covalent bonds.
- Substances that consist of simple molecules do not conduct electricity because the molecules do not have an overall electric charge.
- Atoms that share electrons can also form giant structures or macromolecules. Diamond and graphite (forms of carbon) and silicon dioxide (silica) are examples of giant covalent structures (lattices) of atoms. All the atoms in these structures are linked to other atoms by strong covalent bonds and so they have very high melting points. Students should be able to recognise other giant structures or macromolecules from diagrams showing their bonding.
Mapa mental
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Nota de la lección
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- Why does solid sodium chloride not conduct electricity? A. It contains no ions B. Its ions cannot move C. Its electrons are held in covalent bonds D. It contains no charged particles Answer: B
- Solid iodine melts at 114 C. What is overcome when it melts? A. The covalent bonds inside each iodine molecule B. The forces of attraction between iodine molecules C. The attraction between positive and negative ions D. The bonds between the layers of a giant structure Answer: B
- Silicon dioxide melts at 1610 C. Why is its melting point so high? A. It is made of large molecules with strong forces between them B. Its ions are held together in a lattice C. Covalent bonds run right through the structure and must be broken D. Its molecules carry an overall electric charge Answer: C
- Which substance conducts electricity when molten but not when solid? A. Iodine B. Potassium bromide C. Silicon dioxide D. Methane Answer: B
- Diamond and silicon dioxide both have very high melting points. What do their structures have in common? A. Both consist of simple molecules held by weak forces B. Both are lattices of oppositely charged ions C. In both, strong covalent bonds link the atoms throughout the structure D. In both, every atom has one electron that is free to move Answer: C
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