Year 9 Science · Unit 2 · Lesson 9
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Learning Goals
1. Complete the table. An atom usually forms one covalent bond for each electron it needs to fill its outer shell.
| Atom | Outer-shell electrons | Electrons needed | Usual number of covalent bonds |
|---|---|---|---|
| Hydrogen | 1 | 1 | 1 |
| Carbon | |||
| Nitrogen | |||
| Oxygen | |||
| Chlorine |
2. Show how electrons are shared in (a) a water molecule, H₂O, and (b) an oxygen molecule, O₂. Use dots for one atom’s electrons and crosses for the other’s, and circle each shared pair.
Compare two
Complete the table for CO₂ and diamond.
| Feature | CO₂ | Diamond |
|---|---|---|
| Structure | ||
| What must be overcome to melt or turn it to gas | ||
| State at room temperature | ||
| Number of covalent bonds on each carbon |
3. Chip packets are often filled with nitrogen gas instead of air, so the chips do not go stale. Nitrogen’s triple bond needs 945 kJ/mol to break, compared with 498 kJ/mol for oxygen’s double bond. Use this to explain why nitrogen is a good choice.
Data: the halogens
Fluorine, chlorine, bromine and iodine each form two-atom molecules (F₂, Cl₂, Br₂, I₂) with one shared pair.
| Halogen | Melting point (°C) | Boiling point (°C) | State at 25 °C |
|---|---|---|---|
| Fluorine, F₂ | −220 | −188 | |
| Chlorine, Cl₂ | −101 | −34 | |
| Bromine, Br₂ | −7 | 59 | |
| Iodine, I₂ | 114 | 184 |
Source: data book values, rounded to the nearest degree.
4. Use the melting and boiling points to fill in the state of each halogen at 25 °C. Describe the trend down the group.
5. When bromine boils at 59 °C, is the Br–Br covalent bond broken? Explain, and suggest why the halogens with bigger molecules boil at higher temperatures.
6. Sulfur and silicon are both elements whose atoms are joined by covalent bonds. Sulfur is made of S₈ molecules and melts at 115 °C. Silicon, used to make computer chips, is a giant covalent structure and melts at 1414 °C. Explain the difference in their melting points.