17. Simple Molecular vs. Giant Covalent Structures

1. Simple Molecular Substances

These are small molecules made of just a few non-metal atoms (e.g., Chlorine, Ammonia).

• Bonding: Atoms within each molecule are joined by strong covalent bonds. However, the separate molecules are only held together by weak intermolecular forces.

• Melting and Boiling Points: They have low melting and boiling points because you only need to break the weak intermolecular forces to change state, not the strong covalent bonds.

• Trends: As molecules get larger (e.g., going down Group 7), there are more intermolecular forces, so the melting and boiling points increase.

• Conductivity: They do not conduct electricity because they have no free electrons or ions, and the molecules themselves have no charge.

2. Giant Covalent Structures

These are made of huge numbers of non-metal atoms all bonded to each other in regular repeating lattices (e.g., Diamond, Graphite, Silicon Dioxide).

• Bonding: All atoms are joined by strong covalent bonds.

• Melting and Boiling Points: They have very high melting and boiling points because you must break these strong covalent bonds to melt them, which requires a lot of energy.

• Strength: They are very strong and rigid due to the massive network of covalent bonds.

• Conductivity: Most do not conduct electricity because they don't contain charged particles (Graphite is a notable exception).

3. Silicon Dioxide (Silica)

• Composition: Made of silicon and oxygen atoms in a 1:2 ratio (SiO2).

• Fact: It is the main component of sand.

• Structure: It is a giant covalent structure, meaning it is very strong and has a high melting point.

Summary Table