Molecules – A molecule is the smallest particle of a pure chemical substance that retains its characteristic composition and its characteristic chemical properties. The size of a molecule can vary widely from a small particle consisting, perhaps of two or three atoms that is completely invisible to the naked eye to an immense DNA molecule that can attain microscopic size (so called macromolecules). The smallest molecule, a molecule of hydrogen (two hydrogen atoms) has a length of only 0.74Å.  The atomic composition of a molecule does not define a molecule uniquely as the same atoms may be present but arranged differently – these are called molecular isomers. The ultimate proof of the existence of molecules was due to the work of Perrin on Brownian movement, which in fact, proves the existence of motion that is due to molecular movement.

Further Information on this Topic can be found in the books of R.P.W. Scott by following the listed links