The physical properties of simple molecular substances can be explained by thinking about their structure and bonding.
There are intermolecular forces between simple molecules . Intermolecular forces are much weaker than the strong covalent bonds within the molecules. When simple molecular substances melt or boil, it is these weak intermolecular forces that are overcome. The covalent bonds are not broken. Very little energy is needed to overcome the intermolecular forces, so simple molecular substances usually have low melting and boiling points. They are often liquids or gases at room temperature.
The more energy needed, the higher the melting point or boiling point.
For example, the longer the alkane molecule, the higher the boiling point.
A substance can conduct electricity if:
Simple molecules have no overall charge, or charged particles that can separate, so simple molecular substances cannot conduct electricity, even when liquid or dissolved in water.