Describe metallic bonding and the "sea of electrons" model, and compare the relative strength of intermolecular forces (dispersion, dipole-dipole, and hydrogen bonding).
A third type of bonding, metallic bonding, occurs between atoms of the same metal. Instead of transferring or sharing electrons between specific pairs of atoms, metal atoms release their valence electrons into a shared, mobile "sea of electrons" that flows freely around a fixed lattice of positive metal ions. This single model explains several metal properties at once: electrical conductivity (the free-flowing electrons carry charge easily), malleability and ductility (the metal ions can slide past each other into new positions without breaking any specific bond, since the electron sea holds everything together no matter how the ions rearrange), and metallic luster (the mobile electrons interact with and reflect light).
Intermolecular forces (IMFs) are attractions BETWEEN separate molecules, and they are meaningfully weaker than the bonds (ionic, covalent, or metallic) holding atoms together WITHIN a single molecule. IMFs are still important, though, since they are what determine a molecular substance's melting point, boiling point, and physical state at room temperature.
There are three main kinds of IMFs, from weakest to strongest. London dispersion forces exist between ALL molecules (even nonpolar ones), caused by momentary, random shifts in electron position creating brief, temporary charges. Dipole-dipole forces occur between polar molecules, where the permanent slightly-positive end of one molecule attracts the permanent slightly-negative end of another. Hydrogen bonding is an especially strong, specific type of dipole-dipole force that occurs only when hydrogen is directly bonded to nitrogen, oxygen, or fluorine -- and it is the reason water has such an unusually high boiling point for such a small molecule, since a huge amount of extra energy is needed to break all of those hydrogen bonds between water molecules before boiling can occur.
Mastery looks like: They describe the sea-of-electrons model of metallic bonding and correctly rank and identify intermolecular forces (dispersion, dipole-dipole, hydrogen bonding) for a given molecule.
Common stumbles: Confusing intermolecular forces with the covalent bonds inside a molecule; forgetting that hydrogen bonding requires hydrogen attached specifically to N, O, or F, not just any polar bond.