Chapter 3: Atoms and Molecules Test Test Time Explore various categories of non-fiction books. Laws of Chemical Combination Law of conservation of mass and law of constant proportions explained with examples. What is an Atom? Basic definition of atom as the smallest particle of an element that retains its properties. What is a Molecule? Definition and examples of molecules of elements and compounds. Atomicity The number of atoms present in a molecule — monoatomic, diatomic, triatomic, etc. Ions and their Types Definition of ions, types of ions — cations and anions, and their formation. Chemical Formulae Rules for writing chemical formulae of compounds using valency. Molecular Mass and Formula Unit Mass Calculation of molecular mass and formula unit mass using atomic masses. Mole Concept Definition of a mole, Avogadro's number, and use in calculating number of particles. Converting Units using Mole Concept Conversions between grams, moles, and number of particles using molar mass.