Chemical reasoning, as used both in applications and in basic research, resembles a detective story in which tangible clues lead to a mental picture … - David W. Oxtoby

Quantum mechanics explains the physical stability of the atom by predicting its allowed discrete energy levels and defining the wave functions (also called atomic orbitals) associated with each energy level. The orbitals determine the probability density for finding the electrons at particular locations in the atom when the electrons are in a specific quantum state.

Investigating chemical reactions can be greatly complicated and often obscured by the presence of extraneous materials. So, the first step, therefore, is to learn how to analyze and classify materials to ensure that you are working with pure substances before initiating any reactions.

The definite mass ratios involved in reactions suggested a convenient method for counting the number of atoms of each element participating in the reaction. These results, summarized as the laws of chemical combination,provided overwhelming, if indirect, evidence for the existence of atoms and molecules.