Explain in terms of structure and bonding the properties of simple molecular compounds: (a) low melting points and boiling points in terms of weak intermolecular forces (specific types of intermolecular forces are not required) (b) poor electrical co

Atoms, Elements and Compounds – Simple Molecules and Covalent Bonds

What are Simple Molecular Compounds? 🧪

Simple molecular compounds are made of covalently bonded atoms that form distinct molecules. They are usually non‑metallic and have the general formula \$AnBm\$. Because the atoms share electrons, the molecules are neutral and do not contain free ions.

Why do they have low melting and boiling points? 🔬

Think of molecules as people at a party. If everyone only holds hands loosely, they can easily walk away from each other. In chemistry, the “holding hands” are intermolecular forces—weak attractions between molecules. Because these forces are weak:

• Only a small amount of heat is needed to break them, so the melting and boiling points are low.
• Smaller molecules (e.g., \$CH4\$, \$NH3\$) have even fewer forces to overcome, so they melt or boil at even lower temperatures.
• Large, branched molecules can have more surface area, slightly increasing the forces and raising the points a bit.

Why do they conduct poorly? ⚡️

Electrical conductivity requires free charge carriers (ions or electrons). In simple covalent molecules:

1. There are no free ions in the solid or liquid state.
2. Electrons are tightly bound within the covalent bonds and do not move freely.
3. Only when the molecules are dissolved in water and dissociate into ions (e.g., \$NaCl\$) does good conductivity occur.

Thus, simple molecular compounds are poor conductors of electricity.

Examples & Summary Table 📊

Remember: Low melting/boiling points = weak intermolecular forces. Poor conductivity = no free ions or electrons. These features are key to understanding the behaviour of simple covalent molecules in everyday life.