Substances like sulfur, phosphorus, water, oxygen, nitrogen, carbon dioxide, iodine, chlorine, all the hydrocarbons, ect, exist as simple molecules in all 3 states of matter. Inside the molecule, the atoms are held together by strong covalent bonds and all the electrons are held strongly between the atoms (ie. they are not delocalised). Between neighbouring molecules there are much weaker forces of attraction, called intermolecular forces.
On the basis of this information, predict the properties of covalent molecular substances in terms of MP, BP, state at RT, conductivity and hardness. Justify your predicitions using the examples above.
Unlike covalent bonds, forces between molecules aren't actually bonds, but are merely forces of attraction between molecules, or intermolecular forces. These intermolecular forces affect the behavior of substances - solubility, or phase at a given temperature.
Substances that are non-polar only have London Dispersion Forces - caused by instantaneous imbalance in electron density - and hence, non-polar molecules don't have a strong force of attraction between them. This results to lower BP and MP, and hence being at a gaseous phase at RT. They are also not conductive. Take for example hydrogen (H2). Since the bond involved is an H-H bond, there will be no net positive or net negative charge (no difference in electronegativity). This molecule is non-polar. It's a gas at RT.
Other intermolecular forces include dipole-dipole interactions (e.g. water). This is much stronger, and hence, water is a liquid at RT. In fact, water's dipole-dipole IMF is special, it is a H-bond (happens when H is attached to an highly electronegative species).