 Bogaard (1978), Scerri (1991, 1994) and Hendry (1998) have all questioned the possibility of fully reducing chemical theories about atoms and molecules to quantum mechanics. Bogaard argues that many key chemical concepts such as valence and bonding do not find a natural home in quantum mechanics. In a similar spirit, Scerri points out that the quantum mechanical calculations of atomic spectra standardly presented in chemistry textbooks make highly idealized assumptions about the structure of many-electron systems. These approximations are well-motivated on pragmatic grounds. However, they do not allow quantum mechanics to “approximately reduce” chemical facts, because the errors introduced by these approximations cannot be estimated (Scerri 1991, 1994). Further, one of the most important chemical trends, the length of periods in the Periodic Table, cannot be derived from quantum mechanics, unless experimentally derived chemical information is specifically introduced (Scerri 1997). Drawing on the work of Woolley (1978) and Primas (1981), Hendry (1998) argues that there are principled difficulties in accommodating molecular shape within quantum mechanics: the Born-Oppenheimer approximation effectively adds structure by hand. Although quantum chemistry can be extremely illuminating, these authors argue that it has not reduced chemistry to physics.