Question

How does molecular shape and structure give rise to physical properties. (e.g, phase, viscosity, boiling point, etc.) of a bustance.

Answer 1

The covalent bonds which hold the molecules together are very strong, but these are not related to the physical properties of the substance. Physical properties are governed by the intermolecular forces - forces attracting one molecule to its neighbours which are van der Waals forces or hydrogen bonds. Molecular substances tend to be gases, liquids or low melting point solids, depending on the intermolecular forces of attraction. You don't have to break any covalent bonds in order to melt or boil a molecular substance. The magnitude of the melting or boiling point will depend on the strength of the intermolecular forces. The presence of hydrogen bonding will lift the melting and boiling points. The larger the molecule the more van der Waals attractions are possible - and those will also need more energy to break.

Typically for liquid the viscosity increases with increasing intermolecular forces. Shape of a molecule also affect it's viscosity. Molecules with many branches or kinks will be harder to "slide-by" one another, have higher viscosity than small "round" molecules.

Particles in a solid are tightly packed together and often arranged in a regular pattern; in a liquid, they are close together with no regular arrangement; in a gas, they are far apart with no regular arrangement. Small atoms or molecules give rise to smaller surface area having weaker dispersion forces and take higher ebnergy to break that means they have higher boiling points or melting points.Dispersion forces that develop between atoms in different molecules can attract the two molecules to each other. The forces are relatively weak, however, and become significant only when the molecules are very close. Molecules consisting of mainly larger and heavier atoms and molecules are gases as they exhibit stronger dispersion forces than do smaller and lighter atoms and molecules which produce solid or liquid.