Specification Point 1.47:
Explain why substances with a simple molecular structures are gases or liquids, or solids with low melting and boiling points the term intermolecular forces of attraction can be used to represent all forces between molecules

Simple Molecular Structure

Simple molecular structure: Consists of molecules in which the atoms are joined by strong covalent bonds.

Intermolecular forces of attraction: All forces between molecules.

  • All of these substances have very strong covalent bonds between the atoms, but much weaker forces holding the molecules together.
  • When one of these substances melts or boils, it is these weak ‘intermolecular forces’ that break, not the strong covalent bonds.
  • Because the weak intermolecular forces break down easily these substances have low melting and boiling points.
  • This means simple molecular substances are gases, liquids or solids with low melting points, and low boiling points.

Examples of Simple Molecular Structures:

Simple molecular structures, Edexcel IGCSE Chemistry

The structures of simple molecules

Specification Point 1.48:
Explain why the melting and boiling points of substances with simple molecular structures increase, in general, with increasing relative molecular mass

Melting & Boiling Points

Relative Molecular Mass: Calculated by adding together the relative atomic mass for all the atoms shown in its chemical formula.

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 (positive correlation).
  • An increase in the relative molecular mass of a substance means that there are more covalent bonds in the structure, so there are more intermolecular forces of attraction that need to be broken down.
  • So larger amounts of heat energy are needed to overcome these forces, causing the compound to have a higher melting and boiling point.

Example:

Specification Point 1.51:
Know that covalent compounds do not usually conduct electricity

Covalent Compounds

Covalent Compound: Compound with bonds between non-metal and non-metal formed by the shared pair of electrons between the two atoms.

Electrical Conductivity of Covalent Compounds:

  • Covalent compounds do not conduct electricity as they do not have delocalised electrons that are free to move to conduct an electric charge.

Example: Water ( H2O  )

Dot & cross water, CIE IGCSE Chemistry

Water does not conduct electricity as all valence electrons are used in forming covalent bonds, so there are no delocalised electrons that are free to move to conduct an electric charge

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Author: Jamie

Jamie got a First class degree in Chemistry from Oxford University before going on to teach chemistry full time as a professional tutor. He’s put together these handy revision notes to match the Edexcel IGCSE Chemistry specification so you can learn exactly what you need to know for your exams.