What is the difference between an atom, a molecule, an element and a compound?
Two of these describe particles; two describe substances. Mixing the two categories is the source of nearly all the confusion.
The particle terms:
Atom — the smallest unit of an element that retains its chemical identity. A nucleus of protons and neutrons with electrons around it. What makes it a particular element is the number of protons — nothing else.
Molecule — two or more atoms chemically bonded together. They may be the same element (O₂, the oxygen we breathe) or different elements (H₂O).
The substance terms:
Element — a substance made of only one kind of atom. Hydrogen, carbon, gold. There are around 118, of which the heaviest are synthetic and fleeting.
Compound — a substance made of two or more elements chemically bonded in fixed proportions. Water is always two hydrogen to one oxygen; that ratio is not negotiable.
The two distinctions that clear up the confusion:
A molecule is not necessarily a compound. O₂ is a molecule but not a compound — it is a single element. This trips people up constantly.
A compound is not necessarily molecular. Sodium chloride is a compound, but it is not made of discrete molecules. It is an ionic lattice — a repeating three-dimensional arrangement of sodium and chloride ions. The formula NaCl expresses a ratio, not a particle. Metals and network solids like diamond are similar: no molecules, just extended structures.
Mixture — the fourth term, and the one that completes the picture. Substances physically combined but not chemically bonded, in any proportion, separable by physical means. Air is a mixture; seawater is a mixture; brass is a mixture (an alloy).
The practical test: if the components can be separated by filtering, distilling or magnetism, it is a mixture. If separating them requires a chemical reaction, it is a compound.
Ions are atoms or molecules with a charge from losing or gaining electrons, and isotopes are atoms of the same element with different neutron counts.