Atoms and Molecules - Molecules and Ions

A molecule is the smallest particle of an element or a compound that is capable of an independent existence and shows all the properties of that substance. Examples: $H_2$, $O_2$, $H_2O$.

Atomicity is the number of atoms constituting a molecule. For example, Argon ($Ar$) is monoatomic, Oxygen ($O_2$) is diatomic, and Phosphorus ($P_4$) is tetra-atomic.

Molecules of compounds contain atoms of different elements joined together in definite proportions by mass. For example, in $H_2O$, the ratio by mass of $H:O$ is $1:8$.

An ion is a charged particle and can be negatively or positively charged. A negatively charged ion is called an anion ($Cl^-$), and a positively charged ion is called a cation ($Na^+$).

Polyatomic ions are groups of atoms carrying a fixed charge, such as Nitrate ($NO_3^-$), Ammonium ($NH_4^+$), or Carbonate ($CO_3^{2-}$).

The chemical formula of a compound is a symbolic representation of its composition. The valencies or charges on the ions must balance to result in a neutral compound.

Molecular mass is the sum of the atomic masses of all the atoms in a molecule of the substance. It is expressed in atomic mass units ($u$).