Simple Molecular Structures (Edexcel IGCSE Chemistry (Modular))

Simple molecular structures

• Simple molecular structures have covalent bonds joining the atoms together, but intermolecular forces that act between neighbouring molecules

• They have relatively low melting and boiling points because: 

  • There are weak intermolecular forces between the molecules

  • These forces require little energy to overcome 

• Most simple molecular structures are either gases or liquids at room temperature 

• They can be solids with low melting and boiling points but this is less common

• As the molecules increase in size, the melting and boiling points generally increase because the strength of these intermolecular forces increases and so more energy is needed to break them 

Covalent bonds are strong but intermolecular forces are weak

Melting and boiling point patterns

• 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

The relationship between molecular mass and boiling point 

As the molecular mass increases, so does the boiling point

Conductivity of simple molecular structures

Simple molecular structures are poor conductors of electricity (even when molten) because:

• There are no free ions or electrons to move and 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