All matter is composed of ultimately small particles, called atoms. Atoms are permanent and indivisible - they can neither be created nor destroyed. Elements are characterized by their atoms. All atoms of a given element are identical in all respects. Atoms of different elements have different properties.
Chemical change consists of a combination, separation, or rearrangement of atoms.
Chemical compounds are composed of atoms of two or more elements in fixed ratios. All of these statements are close to reality, and nearly describe chemical behavior. But there are exceptions. Thus atoms can be created and destroyed via nuclear processes. They consist of different forms called isotopes.