 Microstructuralism in the philosophy of chemistry is the thesis that chemical kinds can be individuated solely in terms of their microstructural properties (Hendry 2006). As exemplified above by the case of iron, the chemical elements provide paradigmatic kinds that may be individuated microstructurally, since the atomic number—the number of protons in the nucleus—suffices to identify the element. It is true that macro-level chemical and physical properties can also serve to individuate chemical elements, and in the nineteenth century chemists were able to individuate elements without knowing what we do about nuclear structure. However, it is held that atomic number has explanatory priority: the number of protons in the nucleus, and hence the nuclear charge, explains the structure of electrons surrounding the nucleus, which, in turn, explains the chemical behaviour of the element.