Chemical Bonding
Senior 2 · Chemistry
Chemical Bonding
Lesson 4 of 4
22 min
Diamond and graphite are both made of nothing but carbon atoms. Diamond is the hardest natural substance and is used on the tips of drills that cut rock; graphite is so soft that it rubs off onto paper as pencil "lead". Sand (silicon dioxide) melts at about 1700 °C and is used to make glass, yet carbon dioxide, from the element just above silicon, is a gas.
The differences come from structure: the way the particles are arranged and held together. National examinations often ask you to "explain in terms of structure and bonding". By the end of this lesson you will be able to identify the four main structures and predict a substance's properties from its structure, or its structure from its properties.
| Structure | Particles | Held together by | Examples |
|---|---|---|---|
| Giant ionic | Positive and negative ions | Ionic bonds throughout the lattice | NaCl, MgO, CaCO3 |
| Simple molecular | Small molecules | Strong covalent bonds within each molecule; weak forces between molecules | H2O, CO2, CH4, I2, sugar |
| Giant covalent | Atoms | Covalent bonds throughout the whole structure | Diamond, graphite, silicon dioxide (SiO2) |
| Giant metallic | Positive ions and delocalised electrons | Metallic bonds throughout the lattice | Copper, iron, aluminium |
In a giant covalent (covalent network) structure, every atom is joined to its neighbours by covalent bonds, on and on through the whole crystal. To melt it, you must break covalent bonds, which are strong, so these substances have very high melting points.
| Property | Giant ionic | Simple molecular | Giant covalent | Giant metallic |
|---|---|---|---|---|
| Melting point | High | Low | Very high | Usually high |
| Conducts when solid? | No | No | No (graphite: yes) | Yes |
| Conducts when molten or dissolved? | Yes | No | No | Yes (molten) |
| Solubility in water | Usually soluble | Usually insoluble | Insoluble | Insoluble |
| Substance | Melting point | Conducts as a solid? | Conducts when molten? |
|---|---|---|---|
| P | 801 °C | No | Yes |
| Q | −23 °C | No | No |
| R | 1085 °C | Yes | Yes |
| S | above 3500 °C | No | No |
Solution.
Tip
"Explain the melting point" in three moves. (1) Name the structure. (2) Name the force that must be overcome and say whether it is strong or weak. (3) Link it to energy: "many strong covalent bonds must be broken, which needs a lot of energy". For simple molecular substances, say that the weak forces between molecules are overcome and the covalent bonds do not break. Saying "the covalent bonds break" when water boils loses the mark.
Try each item before you read the solution.
Explain why graphite conducts electricity but diamond does not.
Solution: In graphite each carbon uses only three of its outer electrons in bonds; the fourth is delocalised along the layers and can move, carrying a current. In diamond all four outer electrons are held in covalent bonds, so there are no free electrons.
Silicon dioxide melts at about 1700 °C, but carbon dioxide is a gas at room temperature. Explain the difference.
Solution: Silicon dioxide is giant covalent: melting it means breaking many strong Si–O covalent bonds. Carbon dioxide is simple molecular: only the weak forces between CO2 molecules must be overcome, which needs very little energy.
Sugar dissolves in water, but the solution does not conduct electricity. What does this tell you about sugar's structure?
Solution: Sugar is a simple molecular (covalent) substance. It dissolves as whole, uncharged molecules, not ions, so the solution contains no charged particles that can move and carry a current.
Predict the type of bonding in (a) potassium oxide, (b) hydrogen sulfide, (c) zinc.
Solution: (a) Ionic (metal + non-metal). (b) Covalent, simple molecular (non-metal + non-metal). (c) Metallic (a metal element).
Summary
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