Intermolecular Forces
While bonds hold atoms together within a molecule, intermolecular forces are the weaker attractions between separate molecules — and they're what determine properties like boiling point.
Hydrogen bonding is the strongest common intermolecular force, occurring when hydrogen is bonded to a highly electronegative atom like oxygen or nitrogen. Weaker London dispersion forces exist between all molecules, even non-polar ones.
Ionic and Covalent Bonding
An ionic bond forms when one atom transfers electrons to another, creating oppositely charged ions that attract each other — typically between a metal and a non-metal, like in sodium chloride.
A covalent bond forms when two atoms share a pair of electrons instead of transferring them, which is common between two non-metals. Shared electrons let both atoms count them towards a stable outer shell.