Properties of Simple Molecular Compounds
Section: 2. Atoms, Elements & Compounds | Syllabus: Cambridge AS Level Physics 9702
Overview Simple molecular compounds have characteristic physical properties that can be explained by their structure and bonding. Understanding the difference between strong covalent bonds within molecules and weak forces between molecules is key.
Key Concept Simple molecular compounds have strong covalent bonds holding atoms together within each molecule, but only weak intermolecular forces between separate molecules. This explains their properties.
Property 1: Low Melting and Boiling Points Intermolecular Forces Weak forces of attraction between separate molecules. Much weaker than the covalent bonds within molecules. Explanation: Simple molecules are held together by weak intermolecular forces These forces are much weaker than covalent bonds or ionic bonds Only a small amount of energy is needed to overcome these weak forces When melted or boiled, only the forces between molecules break The covalent bonds within molecules remain intact Therefore, simple molecular compounds have low melting and boiling points Important When simple molecular substances melt or boil, the molecules separate from each other but the molecules themselves do NOT break apart.
The covalent bonds within molecules stay strong. Substance Formula Melting Point (°C) Boiling Point (°C) State at Room Temp Hydrogen H₂ -259 -253 Gas Oxygen O₂ -218 -183 Gas Nitrogen N₂ -210 -196 Gas Chlorine Cl₂ -102 -34 Gas Water H₂O 0 100 Liquid Methane CH₄ -182 -164 Gas Carbon dioxide CO₂ -57 (sublimes) -78 (sublimes) Gas Compare with Ionic Ionic compounds like NaCl have melting points of 801°C.
Simple molecular compounds have much lower melting points (often below 100°C) because intermolecular forces are much weaker than ionic bonds. Property 2: State at Room Temperature Because of their low melting and boiling points, most simple molecular substances are: Gases at room temperature (e.g., H₂, O₂, N₂, Cl₂, CO₂, CH₄) Liquids at room temperature (e.g., H₂O, ethanol, bromine Br₂) Rarely solids at room temperature (e.g., iodine I₂, glucose) Explanation Room temperature (about 20-25°C) is often above the boiling point of many simple molecular substances, so they exist as gases.
The weak intermolecular forces are easily overcome by thermal energy at room temperature. Property 3: Electrical Conductivity Simple molecular compounds do NOT conduct electricity Explanation Why: Simple molecular substances do not contain charged particles (ions or free electrons) that can move and carry electric charge.
Result: They cannot conduct electricity in any state - solid, liquid, or gas. Molecules are neutral - no overall charge No free electrons available to carry charge No ions present Therefore, no electrical conduction in any state State Ionic Compounds Simple Molecular Compounds Solid Do NOT conduct (ions fixed) Do NOT conduct (no charged particles) Molten/Liquid DO conduct (ions free to move) Do NOT conduct (still no charged particles) Aqueous DO conduct (ions free in solution) Usually do NOT conduct (molecules stay neutral) Exception Some molecular substances like HCl dissolve in water and react to form ions (H⁺ and Cl⁻), so the resulting solution CAN conduct electricity.
But pure HCl gas or liquid does not conduct. Property 4: Solubility Solubility in Water Simple molecular substances have varying solubility in water: Most are insoluble or only slightly soluble in water Examples of insoluble: O₂, N₂, CH₄, most hydrocarbons Some polar molecules do dissolve in water (e.g., NH₃, HCl, ethanol) Water is a polar solvent - dissolves polar substances well Rule of Thumb "Like dissolves like" - Polar molecular substances tend to dissolve in polar solvents (like water).
Non-polar molecular substances dissolve better in non-polar solvents (like hexane or oil). Solubility in Organic Solvents Many simple molecular substances dissolve well in organic solvents Examples of organic solvents: hexane, ethanol, acetone Non-polar molecules dissolve well in non-polar organic solvents Property 5: Hardness and Strength Simple molecular substances that are solid at room temperature tend to be: Soft and easily broken Can be crushed or scratched easily Examples: iodine crystals, solid carbon dioxide (dry ice) Explanation The weak intermolecular forces between molecules are easily broken, so molecular solids are typically soft and fragile.
Compare this to ionic solids (hard but brittle) and giant covalent structures (very hard). Summary of Properties Property Observation Explanation Melting/Boiling Point Low (often below 100°C) Weak intermolecular forces require little energy to overcome State at Room Temp Usually gas or liquid Room temperature is above melting/boiling point for many Electrical Conductivity Does not conduct No charged particles (ions or free electrons) to carry current Solubility in Water Variable - most insoluble Depends on polarity; "like dissolves like" Hardness (if solid) Soft, easily broken Weak intermolecular forces easily overcome Comparing Structure Types Property Sim…
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