Forces in solids and liquids
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Forces in Solids and Liquids. Types. Strong forces – ionic bonds between ions (metal and non-metal atoms), covalent bonds (C and Si structures), metallic bonds (metal atoms only) Weak forces – between MOLECULES (non-metal atoms only). The nature of intermolecular forces.

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  • Strong forces – ionic bonds between ions (metal and non-metal atoms), covalent bonds (C and Si structures), metallic bonds (metal atoms only)

  • Weak forces – between MOLECULES (non-metal atoms only)

The nature of intermolecular forces
The nature of intermolecular forces

  • Intermolecular forces are the forces that hold MOLECULES together in a MOLECULAR SOLID or LIQUID.

  • The forces are WEAK in comparison to covalent, metallic and ionic bonds – this explains why molecular solids have LOW melting and boiling points

The three types
The three types

  • Hydrogen bonding

    Occurs only between molecules that contain a F, O or N bonded to a H

  • Permanent dipole-permanent dipole

    Occurs between molecules which are polar

  • Temporary dipole-temporary dipole

    Occurs between all molecules. Is significant for non-polar molecules and large molecules


For each compound state the type of force holding the particles together. If molecular, draw its structure and state the most significant type of WIF


Bromine melts at -7°C, and sodium bromide melts at 755°C. Explain why there is a big difference in their melting points.


Bromine is a molecular solid with molecules composed of 2 covalently bonded bromine atoms. The molecules are held together by temporary dipoles as Br2 is a non-polar molecule. It is non-polar because the both Br atoms have identical electronegativity. Temporary dipoles are relatively weak, therefore little energy is required to break them and the melting point is low.

NaBr is an ionic solid composed of Na+ and Br- ions in a 3D lattice. The ionic bonds holding the ions together are strong, requiring a lot of energy to break and therefore the melting point is high.

Question complete and hand in for marking
Question: (complete and hand in for marking)

Rank the following compounds from highest to lowest melting point. Explain your reasoning:

Water diamond carbon dioxide sulfur dioxide sulfur trioxide iron


Highest to lowest:

Diamond - strong covalent bonds between C atoms

Iron – strong metallic bonds between Fe atoms

Water – WIF – hydrogen bonds between H2O molecules (contains O bonded to H)

Sulfur dioxide – WIF – permanent dipoles between SO2 molecules (polar)

Sulfur trioxide – WIF – temporary dipoles between SO3 molecules (non-polar)

Carbon dioxide - WIF – temporary dipoles between CO2 molecules (non-polar and fewer electrons than SO3)