Simple Molecular Structures
Melting and Boiling Points: Simple molecular substances typically have low melting and boiling points because only the weak intermolecular forces must be overcome to change state. Since these forces require very little energy to break, most of these substances exist as gases or liquids at room temperature.
Molecular Mass Correlation: As the relative molecular mass of a substance increases, the number of electrons within the molecule also increases. This leads to stronger intermolecular forces of attraction, meaning more thermal energy is required to separate the molecules, resulting in higher boiling points.
State at Room Temperature: Small molecules like oxygen () or methane () are gases because their intermolecular forces are extremely weak. Larger molecules, such as those found in heavy oils or waxes, may be liquids or soft solids because their cumulative intermolecular attractions are stronger.
Structure of Buckminsterfullerene: is a unique simple molecular structure consisting of 60 carbon atoms arranged in a hollow sphere of pentagons and hexagons. Although it is made of carbon (like diamond or graphite), it behaves as a simple molecule because the spheres are not covalently bonded to each other.
Physical Properties of : Because there are only weak intermolecular forces between the individual 'buckyballs,' the substance has a lower melting point than giant covalent structures and is relatively slippery. It does not conduct electricity effectively because, although it has some delocalized electrons, they are trapped within individual molecules and cannot jump between spheres.
| Feature | Simple Molecular | Giant Covalent |
|---|---|---|
| Bonding | Strong covalent within, weak intermolecular between | Continuous network of strong covalent bonds |
| Melting Point | Low (little energy to break weak forces) | Very High (massive energy to break covalent bonds) |
| Conductivity | Usually insulators (no free charges) | Variable (Graphite conducts; Diamond does not) |
| Examples | Water, Carbon Dioxide, Methane | Diamond, Graphite, Silicon Dioxide |
The 'Force' Trap: When explaining low boiling points, never say 'covalent bonds are weak.' Always specify that the intermolecular forces are weak, while the covalent bonds remain strong and unbroken during boiling.
Mass and Boiling Point: If asked to compare two similar molecules (like alkanes), always check their relative molecular masses. The larger molecule will almost always have the higher boiling point due to increased intermolecular attractions.
Conductivity Justification: When asked why a substance doesn't conduct, your answer must mention the absence of both delocalized electrons and mobile ions. Mentioning only one may result in partial marks.