Edexcel IGCSE Chemistry 复习笔记 1.7.3 Simple Molecular Structures

• Simple molecular structures have covalent bonds joining the atoms together, but intermolecular forces that act between neighbouring molecules
• They have low melting and boiling points as there are only weak intermolecular forces acting between the molecules
• These forces are very weak when compared to the covalent bonds and so most small molecules are either gases or liquids at room temperature
  • Often the liquids are volatile

   

• As the molecules increase in size the intermolecular forces also increase as there are more electrons available
• This causes the melting and boiling points to increase

Covalent bonds are strong but intermolecular forces are weak

Melting and Boiling Point of Simple Compounds in Relation to Molecular Mass

• As the relative molecular mass of a substance increases, the melting and boiling point will increase as well
• An increase in the relative molecular mass of a substance means that there are more electrons in the structure, so there are more intermolecular forces of attraction that need to be overcome when a substance changes state
• So larger amounts of heat energy are needed to overcome these forces, causing the compound to have a higher melting and boiling point
• The family of organic molecules called alkanes show a clear increase in boiling point as the size of the molecule increases

Graph showing the increase in boiling point as the molecular size increases

• They are poor conductors of electricity as there are no free ions or electrons to carry the charge
• Most covalent compounds do not conduct at all in the solid state and are thus insulators
• Common insulators include the plastic coating around household electrical wiring, rubber and wood

The plastic coating around electrical wires is made from covalent substances that do not allow a flow of charge