Overview
This lecture explains the differences between simple molecular substances and giant covalent structures, focusing on their bonding, structure, and key physical properties.
Simple Molecular Substances
- Nonmetals can form small molecules by sharing electrons through covalent bonds.
- Examples include chlorine and ammonia, which are called simple molecular substances.
- The strong covalent bonds are within the molecules, but weak intermolecular forces exist between them.
- Melting and boiling simple molecular substances only requires breaking weak intermolecular forces, not covalent bonds.
- As molecule size increases (e.g., going down the halogen group), intermolecular forces and boiling points increase.
- Simple molecular substances do not conduct electricity because their molecules lack free electrons or charged particles.
Giant Covalent Structures
- Nonmetals can also form giant covalent structures where all atoms are joined by covalent bonds in regular lattices.
- Examples include diamond, graphite, and silicon dioxide (silica).
- Giant covalent structures are very strong and have high melting and boiling points due to strong bonds throughout the lattice.
- Most giant covalent structures do not conduct electricity because they have no free-moving charged particles.
- An exception is graphite, which can conduct electricity due to its structure.
Silicon Dioxide (Silica)
- Silicon dioxide consists of silicon and oxygen atoms in a 1:2 ratio.
- It is a giant covalent structure and is the main component of sand.
- Recognizing the structure of silicon dioxide is important, but drawing it is not required.
Key Terms & Definitions
- Covalent Bond — A chemical bond where two atoms share electrons.
- Intermolecular Forces — Weak forces acting between separate molecules.
- Simple Molecular Substances — Small molecules held together by covalent bonds, joined to each other by weak intermolecular forces.
- Giant Covalent Structures — Large networks of atoms connected by covalent bonds, forming a repeating lattice.
- Silicon Dioxide (Silica) — A compound with a giant covalent structure made of silicon and oxygen.
Action Items / Next Steps
- Be able to distinguish properties of simple molecular substances and giant covalent structures.
- Learn to recognize the structure and composition of silicon dioxide.