* Molecule: A molecule is formed when two or more atoms bond together. These atoms can be of the same element (like O2, oxygen gas) or different elements (like H2O, water).
* Compound: A compound is formed when two or more *different* elements bond together chemically.
Key Points:
* Discrete Unit: A molecule is a distinct, separate unit that can exist on its own.
* Independent Existence: Molecules can move around and interact with other molecules independently.
Examples:
* H2O (water): This formula represents a single molecule of water, consisting of two hydrogen atoms and one oxygen atom.
* NaCl (sodium chloride): This formula represents a formula unit of sodium chloride, a salt. In a solid state, it exists as a lattice of ions, not individual molecules.
When a formula is *not* a molecule:
* Ionic Compounds: Compounds formed by ionic bonds (like NaCl) usually don't form discrete molecules. They form lattices of ions.
* Giant Covalent Structures: Large networks of atoms held together by covalent bonds (like diamond) do not have distinct molecules.
In summary: A compound's formula represents a molecule when it describes a discrete, independent unit of that compound, formed by the bonding of atoms.
