An experimentalist's reply to "What is an atom in a molecule?".

Parr, Ayers and Nalewajski have opined in this Journal that the concept of an atom in a molecule "is an object knowable by the mind or intellect, not by the senses." This view is countered by the two hundred years of experimental chemistry underlying the realization that the properties of some total system are the sum of its atomic contributions. This paper concludes that an experimentalist has no doubt but that he or she is measuring the properties of atoms when performing an experiment.