Small molecules: weak intermolecular forces set their properties

1. A small molecule is a discrete group of a small number of atoms joined together by strong covalent bonds.
2. Inside each molecule, shared pairs of electrons hold the atoms together very strongly.
3. Familiar examples include $H_2$, $O_2$, $H_{2}O$ and ${CO}_2$.
4. Separate molecules are attracted to one another by intermolecular forces.
5. These intermolecular forces are much weaker than the covalent bonds within the molecules.
   1. This large difference in strength is the key to explaining the bulk properties of molecular substances.
6. Because the forces between molecules are weak, only a small amount of energy is needed to pull the molecules apart.
7. Substances made of small molecules therefore have low melting points and boiling points.
8. Many are gases or liquids at room temperature.

Changes of state: the forces between molecules give way while the molecules stay whole

1. Heating a molecular substance transfers energy to its molecules.
2. At the melting point, enough energy is supplied to overcome some intermolecular forces so the molecules can move past one another.
3. At the boiling point, enough energy is supplied to overcome the intermolecular forces fully so the molecules separate into a gas.
4. Only the weak intermolecular forces are overcome when a molecular substance melts or boils.
5. The strong covalent bonds inside each molecule stay intact.
6. Each molecule therefore contains the same bonded atoms before and after the change of state.

Molecular size: larger molecules mean stronger forces and higher melting points

1. Intermolecular forces become stronger as the molecules become larger.
2. Stronger intermolecular forces need more energy to overcome.
3. Larger molecules therefore usually have higher melting points and boiling points than smaller molecules.
4. This trend is caused by stronger forces between the molecules, not by stronger covalent bonds within them.

Electrical conductivity: neutral molecules cannot carry a current

1. A small molecule has no overall electric charge because its positive and negative charges balance.
2. A molecular substance contains no ions or free electrons that can move through it.
3. Such substances therefore do not conduct electricity in any state.
4. Even as a liquid or gas the molecules can move, but this carries no current because each molecule stays neutral.