Justin, Just replying to answer some of your specific questions, re: periodocity, orbitals, and reduction, although these issues don't exhaust the range of issues just beginning to be considered in the emerging field of philosophy of chemistry. From "The Case for Philosophy of Chemistry" by Serri and McIntyre One very important form of explanation which pervades all areas of chemistry, from teaching to frontier research, lies in talk of electron shells or orbitals, as they are often called. The formation of bonds, acid-base behavior, redox chemistry, photochemistry, reactivity studies, etc., are all regularly discussed by reference to the interchange of electrons between various kinds of orbitals. This approach may at first sight seem to speak in favor of the epistemo- logical reduction of chemistry to physics, since talk of electron shells is thought to belong primarily to the level of atomic physics. However, a more critical examination of the issues involved reveals no such underpinning from fundamental physics. It emerges that explanations in terms of electron orbitals, and indeed all talk of orbitals in chemistry, is not sanctioned by our present understanding of quantum mechanics. The remarkable fact is that at the most fundamental quantum mechanical level electronic orbitals become ontologically redundant. Electronic orbitals simply do not exist according to quantum mechanics, although they remain as a very useful explanatory device. This result is embodied in the more fundamental ver- sion of the Pauli Exclusion Principle, which is frequently forgotten at the expense of the restricted and strictly invalid version of the Principle, which does uphold the notion of electronic orbitals (Scerri 1991, 1995). This situation implies that most explanations given in chemistry which rely on the existence of electrons in particular orbitals are in fact "level specific" explanations, which cannot be reduced to or underwritten by quantum mechanics.18 Thus, a case has been demonstrated where the explanation of what it is that we seek to know when we engage in chemical explanation would seem to suggest that we eschew reductive explanations, and support the explanatory autonomy of chemistry. from "Philosophy of Chemistry?A New Interdisciplinary Field?" by Serri How many of us have experienced students' frustration when we give different chemical explanations depending on the context in which one and the same phenomenon is being discussed? If one believes only in fundamental explanations, this form of activity appears to be seriously mistaken. However, as chemists we are also aware of the need to operate on many levels and the fact that explanations can be genuinely level- specific. Such approaches must be used very carefully. They should not degenerate into the introduction of ad hoc explana- tions that are invoked in the explanation of particular chemical facts but cannot be generalized to other situations. One example is the wide variety of explanations given for the apparent orbital paradox concerning the relative occupation and ionization of the 4s and 3d levels in the first transition metal series. The paradox I allude to is that the 4s orbital is preferentially occupied but also preferentially ionized. Nobody has yet rationalized this situation at a level that might be appropriate for teaching general chemistry. Most educators and textbooks continue to argue that the 4s orbital is prefer- entially occupied because it has a lower energy than 3d, in spite of several articles published in this Journal that state that the 4s orbital never has a lower energy than the 3d (9). Another response, encountered particularly among theo- eticians, is that this is a futile question because the concept of orbitals ceases to refer to any objective entities in more advanced calculations and can only be maintained at the level of the Hartree?Fock approximation. I suggest that this kind of response is just another way of expressing Dirac's famous dictum whereby chemistry has been explained in principle by quantum mechanics. Such a response amounts to evading the issue, which is to try to obtain a consistent explanation within an orbital approximation such as the Hartree?Fock model, since within this regime the concept of an orbital is well defined. JPDeMouy ========================================= Need Something? Check here: http://ludwig.squarespace.com/wittrslinks/