A chemical element is a fundamental substance of the material world. Each element has an identity; for example, gold consists of only gold atoms, and a gold atoms is unlike any other atom. Indeed, a gold atom can be split, but the particles that (electrons, protons, etc.) that constitute a gold atom are not gold. It could be said that subatomic particle are generic, interchangeable. Atoms, on the other hand, have an identity, and constitute the identity of an element.

Chemists use the concept of atomic number to identify particular elements. Atomic number is the number of protons in an atom's nucleus, which means that atomic number constitutes, so to speak, a particular atom's identity tag. Thus, when we ascertain that a particular atom has 11 protons, we can immediately identify it as a sodium atom. No other atom has 11 protons. Unfortunately, despite their clearly defined identity...