Molecules

A molecule is a group of atoms bonded by covalent (electron sharing) bonds. The number of atoms is generally small, but it may be quite large.

Atoms are strongly linked. The molecule can be viewed as a single object, and it is the smallest part of a substance. Molecules may be linked to neighbouring molecules, but forces between molecules are weak, and sometimes they may be neglected.
Molecules and atoms are made of electrically charged subparticles, nuclei an electrons. The sum of (positive and negative) charges must be zero. A molecule contains all the nuclei and the electrons of its constituting atoms.

In some cases, molecule-like particles are formed by atoms with a net electric imbalance (negative for excess electrons or positive for defect electrons). They are called molecular ions and must combine with opposite charge ions to form compounds.

Many substances are made of molecules, but some do not. Non-molecular substances include ionic compounds, metals and covalent solids like diamond.