properties of simple molecular subatance ( covalent bonds )

Electricity

Simple molecular substances are made up of covalent bonds, where there are no ions and no delocalised electrons - meaning simple molecular substances are poor conductors of electricity.

The covalent bonds inside the molecule are strong

It’s the intermolecular forces between molecules that break when melting and boiling

The strength of these intermolecular forces vary between states - but they are all overall weak

We never break covalent bonds when melting, we break the intermolecular forces

Melting & boiling point

Given this, the melting and boiling points are low because the weak intermolecular forces do not require much energy to overcome. However, solid and liquid simple molecular substances are slightly higher than the gases, although still low overall.